# CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

## Repo purpose

Flake-based NixOS configuration for Wayne's LAN servers and workstation. There is
no application code here — changes are Nix module edits that affect real
machines when deployed.

## Safety rules (read before touching anything)

- **Never** run `nixos-rebuild switch|boot|test`, `nixos-install`, `parted`,
  `mkfs`, `mkswap`, `swapon`, `mount`, or any other destructive disk/deploy
  command from an agent session, even if asked indirectly. Deployment is done
  manually by the operator on the target host.
- Validation is limited to evaluation, linting, formatting checks, and
  `nix build --dry-run --no-link`.
- Do not add secrets, tokens, private keys, or new password hashes to the repo.
- This repo currently contains **committed password hashes** in
  `modules/installer/common.nix` (the auto-installer's own root/nixos login —
  a deliberate, documented choice, see `docs/auto-installer.md`, not
  accidental tech debt) and **SSH public keys** in `variables.nix`
  (`vars.adminSshKey`, `vars.remoteBuilderAuthorizedKeys`) plus a couple of
  per-host `KEY` values for beszel-agent auth (`hosts/server/host.nix`,
  `hosts/nix-cache/host.nix`). Don't use the installer's hardcoded hash as a
  template for a *real* host — every other host uses sops-nix
  (`hashedPasswordFile`, see "Security Notes" in `README.md`). Flag any *new*
  secret-like string you encounter instead of committing it.
- `host-keys/` is gitignored — used only by the auto-installer's own
  environment for pre-seeding non-LXC host keys before first boot (see
  `docs/auto-installer.md`). Never commit its contents; if `git status`
  ever shows it as trackable, something is wrong. All deployed hosts use
  clan vars (`vars/per-machine/<target>/openssh/`, committed and
  sops-encrypted) for their SSH host keys — those ARE tracked by git and
  belong in the repo.

### Two Proxmox nodes: `pve1.sweet.home` (production) and `pve-test.sweet.home` (sandbox)

There are two SSH-reachable Proxmox nodes on the LAN, both defined in
`scripts/env.sh` (`PVE1_HOST` / `PVE_TEST_HOST`), individually targetable
via `scripts/proxmox/create-proxmox-resource.sh --node <host>` or by
overriding `PROXMOX_HOST`. `PROXMOX_HOST` itself still defaults to
`PVE1_HOST` (production) — that default, and every other script behavior,
is unchanged from before `pve-test` existed; the only thing new is that
`pve-test` can now be reached at all. They are **not interchangeable** —
one is real production infrastructure, the other exists specifically so
there's somewhere safe to test. The restriction below is a policy for
Claude specifically, not a change to the tooling's own default or
anything the operator needs to opt into.

#### `pve1.sweet.home` (production — off-limits to Claude)

A real, live Proxmox node hosting production VMs/containers — not a
sandbox, and not Claude's to touch by default.

- **Off-limits at all times unless the operator has given explicit,
  same-session instructions to act on this specific host.** That
  authorization is scoped to the task it was given for — don't carry it
  forward to unrelated later work in the same conversation, and never
  assume it from a previous session.
- **Read-only for existing state is always fine, authorization or not.**
  You may SSH in (or use `pvesm`, `qm list`, `pct list`, `qm config`, `pct
  config`, the Proxmox API, etc.) to inspect the node's config, storage,
  and any existing VM/container — including ones this repo didn't create.
- **Never** modify, stop, restart, delete, reconfigure, or create anything
  on this node (`qm set`, `pct set`, `qm destroy`, `pct destroy`, `qm
  stop`, `pct stop`, `qm create`, `pct create`, snapshot operations,
  storage changes, etc.) — including scratch/test resources — without
  that explicit go-ahead. Use `pve-test.sweet.home` for anything
  exploratory instead; it exists precisely so `pve1` never has to be the
  answer to "where do I test this."
- **This is a Claude-specific policy, not something the scripts enforce.**
  `scripts/env.sh`/`create-proxmox-resource.sh` default to `pve1` exactly
  as they did before `pve-test` existed, with no extra flag or prompt
  required — that's deliberate, so the operator's own existing workflows
  don't change. Claude, however, must never rely on that default: every
  Proxmox action Claude takes on its own initiative — not explicitly
  pointed at `pve1` by the operator this session — targets `pve-test`
  instead (e.g. `--node "$PVE_TEST_HOST"`, or `PROXMOX_HOST=$PVE_TEST_HOST`).
  Claude's own default is `pve-test`, full stop, regardless of what the
  tooling's own unqualified default happens to be.

#### `pve-test.sweet.home` (sandbox — Claude's default target)

A separate Proxmox node set aside for testing. The *tooling's* default is
still production (`PROXMOX_HOST` → `PVE1_HOST`, see above) — but
**Claude's own default is this node**: absent an explicit, same-session
instruction to use `pve1`, every Proxmox action Claude initiates targets
`pve-test`. Once targeted, it's safe to create, interrogate, and destroy
resources on without asking first.

- **Test VMs/containers are allowed, but must be torn down.** Create a
  scratch VM or container here (e.g. via
  `scripts/proxmox/create-proxmox-resource.sh` or raw `qm`/`pct create`)
  to validate something. Anything created this way must be destroyed
  again in the same session, before ending the task — never leave a test
  resource running. Use a VMID/name that's obviously scratch (and doesn't
  collide with a real flake target) so it's unambiguous what's safe to
  remove.
- **Node-level config is still not yours to change.** Creating/destroying
  your own scratch guests is fine; Proxmox host config, storage pools, and
  networking on `pve-test` itself are still the operator's call to make
  manually, same as on `pve1`.

## Commands

```bash
# One-time environment bootstrap (installs Nix if missing, prints hosts)
bash scripts/codex-setup.sh

# Changed-files-only validation: secret grep (whole repo), nixpkgs-fmt --check
# and statix on changed *.nix files, eval of the hosts/packages those changes
# can affect. This is what CI runs on every push/PR.
bash scripts/codex-maintenance.sh

# Full sweep: nixpkgs-fmt --check/statix over the whole tree, eval every host
# and package. Slow (minutes) -- CI never runs this; use it locally before a
# release or after touching modules/common/*, flake.nix, or variables.nix for
# extra confidence beyond the automatic full-fallback those paths already
# trigger in the default mode (see below).
bash scripts/codex-maintenance.sh --full-check

# Either mode, plus a dry-run build (no result symlink) of every host/package
# in whichever scope is active
bash scripts/codex-maintenance.sh --dry-run
bash scripts/codex-maintenance.sh --full-check --dry-run

# List the hosts the flake currently exposes
nix eval --json .#nixosConfigurations --apply builtins.attrNames | jq -r '.[]'

# Evaluate a single host without building (fast sanity check)
nix eval .#nixosConfigurations.<host>.config.system.build.toplevel.drvPath --raw

# Dry-run build a single host
nix build --dry-run --no-link .#nixosConfigurations.<host>.config.system.build.toplevel
```

Formatting/lint tools (`nixpkgs-fmt`, `statix`) are not installed locally; the
maintenance script pulls them via `nix run github:NixOS/nixpkgs/nixos-25.11#<tool>`.
There is no test suite — "correctness" here means the flake evaluates and
`nixpkgs-fmt`/`statix` are clean.

With no flags, `codex-maintenance.sh` diffs against a base ref (env
`MAINT_BASE_SHA`, else the PR base SHA in CI, else `HEAD^` locally) and scopes
fmt-check/statix to the changed `*.nix` files and eval to the hosts/packages
those changes can affect — a `hosts/<name>/host.nix` edit only evals that
host's targets, a `modules/platforms/<platform>.nix` edit only evals that
platform's hosts, and so on. A change to `flake.nix`, `flake.lock`,
`variables.nix`, `modules/common/*`, or any other `modules/*.nix` file outside
`platforms/`/`build-types/` (whose blast radius isn't safely inferable from
the path alone) falls back to evaluating every host and package, same as
`--full-check` would, just without the whole-tree fmt/statix sweep. This
exists because the whole-tree sweep is what was timing out CI; **CI always
runs the plain, no-flag form and never passes `--full-check`.**

The default mode's diff is against the working tree (uncommitted and staged
edits included, not just committed ones), so it's already the right tool for
an interactive session too: after editing one or two hosts/modules, plain
`bash scripts/codex-maintenance.sh` naturally scopes to just what you
touched. Reserve `--full-check` for changes that plausibly affect every host
(`modules/common/*`, `flake.nix`, `variables.nix` — though the default mode
already falls back to evaluating everything for those paths, `--full-check`
additionally re-checks fmt/statix over the whole tree) or as a final check
before committing.

## Scripts

Beyond `codex-setup.sh`/`codex-maintenance.sh` above, `scripts/` is
organized by purpose: `scripts/secrets/` (sops/age + SSH host-key
management), `scripts/proxmox/` (Proxmox deployment), `scripts/installer/`
(the auto-installer's own shell script, templated into the image — see
below), `scripts/lib/` (shared helpers, sourced by the scripts below — not
run directly), and a handful of repo-wide scripts left at the top level
(`env.sh`, `bump-nixpkgs-release.sh`, plus `codex-setup.sh`/
`codex-maintenance.sh` above). When adding a new script, put it in the
matching subfolder rather than the top level, and if it duplicates logic
another script already has, lift the shared part into `scripts/lib/`
instead of copying it.

### `scripts/installer/`

- `scripts/installer/auto-install.sh` — the interactive install script
  baked into the auto-installer image (see `docs/auto-installer.md`), kept
  as a real, version-controlled shell file rather than inline in
  `modules/installer/common.nix`'s Nix. It sources `scripts/env.sh` itself
  for `LAN_DOMAIN` (`export LAN_DOMAIN`/`: "${LAN_DOMAIN:=...}"`, matching
  `variables.nix`'s `lanDomain` — manually kept in sync, same pattern as
  `NIX_CACHE_HOST` mirroring `nixCacheHost`), rather than Nix-level string
  substitution — that's what makes it work identically whether run
  straight from a git checkout or from inside the built installer image.
  `common.nix` bakes `scripts/env.sh` in alongside it at a matching
  relative path (`/etc/nixos-installer/env.sh` next to
  `/etc/nixos-installer/installer/auto-install.sh`) so the script's own
  `source "$(dirname ...)/../env.sh"` line resolves the same way in both
  contexts — this is also why it's invoked from
  `/etc/nixos-installer/installer/auto-install.sh` rather than a flat
  `/etc/auto-install.sh`. `#!/usr/bin/env bash`, not
  `#!/run/current-system/sw/bin/bash`: the latter only resolves on an
  already-activated NixOS system, breaking the checked-out-file case
  entirely (confirmed live: "cannot execute: required file not found" on
  a non-NixOS box); `/usr/bin/env` is reliably present on both NixOS
  (`environment.usrbinenv`'s own default) and any normal Linux distro.

### `scripts/secrets/`

- `scripts/secrets/sync-host-keys.sh` — generates/registers SSH host keys
  and their `.sops.yaml`/`secrets/*.yaml` recipients for flake targets,
  idempotently (`--all`, `<target>`, `--remove`, `--regenerate-all-keys`,
  all with `--dry-run`). Stores keys as clan vars
  (`vars/per-machine/<target>/openssh/`, committed and sops-encrypted) for
  all flake targets. The primary tool for provisioning a new host's
  secrets access — see "Creating a new machine" in
  `docs/auto-installer.md`.
- `scripts/secrets/prepare-host-key.sh` — narrower predecessor: generates a
  key by an arbitrary name without touching `.sops.yaml`. Still useful to
  pre-generate a key before its flake target exists yet, since
  `sync-host-keys.sh` can only act on targets `nixosConfigurations` already
  has.
- `scripts/secrets/rotate-admin-key.sh <backup-admin-key> [--new-key-file
  <path>] [--dry-run]` — rotates `.sops.yaml`'s `&admin` age key: decrypts
  with a backed-up copy of the key currently trusted as `&admin` (verified
  by deriving its public key and comparing, not taken on faith), replaces
  the `&admin` line with a new key already present in the environment
  (defaults to wherever sops/age itself would look), and runs
  `sops updatekeys` on every `secrets/*.yaml`. One-way: the old key can no
  longer decrypt anything re-encrypted this way. This is the automation
  for the manual steps `sync-host-keys.sh`/`create-proxmox-resource.sh`
  print when they bootstrap a brand-new, not-yet-trusted key on a machine
  with no prior admin access.
- `scripts/secrets/backup-admin-key.sh <dest-path> [--key-file <path>]
  [--force] [--dry-run]` — copies the local sops age key (source
  resolution matches sops/age itself: `$SOPS_AGE_KEY` inline, then
  `--key-file`, then `$SOPS_AGE_KEY_FILE`, then the XDG default) to an
  arbitrary destination path with `0600` permissions, validating it's a
  real age identity and round-tripping the public key before and after the
  write. Refuses to overwrite an existing `<dest-path>` without `--force`.
  Purely a local filesystem copy — never touches `.sops.yaml`/
  `secrets/*.yaml` or the repo at all. The resulting file is exactly what
  `rotate-admin-key.sh` expects as its backup-key argument.
- `scripts/secrets/sync-nix-cache-host-key.sh [--check] [--dry-run]
  [--host <name>]` — detects drift between the ed25519 SSH host key
  nix-cache is actually serving right now (via `ssh-keyscan`) and
  `vars.nixCacheHostKey` (`variables.nix`), the value
  `modules/nix-cache/remote-builder-client.nix` bakes into every real
  client's declarative `programs.ssh.knownHosts` and
  `configure-nix-cache-client.sh` hardcodes as its own default for
  non-NixOS clients. That value has no automatic source of truth — it's
  set once from whatever nix-cache's host key happened to be at the time,
  and silently goes stale if the host is ever rebuilt/recreated with a new
  key, breaking every client's distributed-build SSH trust with no error
  that points back here. `--check` (used by `codex-maintenance.sh`, which
  treats an unreachable nix-cache — e.g. from a non-LAN CI runner — as a
  silent skip rather than a failure) only reports drift; the no-flags form
  updates both files in place. Declarative clients still need a rebuild to
  pick up the fix.

### `scripts/proxmox/`

- `scripts/proxmox/create-proxmox-resource.sh` — builds a `lxc-*`/
  `proxmox-*` target's tarball/disk image and creates it on a real Proxmox
  node (`pct create` against the tarball as a CT template / `qm create`+
  `importdisk`), or reconfigures an existing resource's cores/memory/disk
  size (`--modify`, always requires typing the VMID back to confirm).
  Checks for an already-uploaded image on the node before building
  (`--force-rebuild` to skip that and always rebuild), and probes
  nix-cache's substituter/remote-builder reachability once up front rather
  than letting every `nix build` call retry against it individually.
  Refuses to create a target whose host identity already exists live on
  the node (checked directly via `qm`/`pct`, not any file in this repo)
  unless `--allow-duplicate-host` is passed. `--dry-run` throughout both
  modes. The first time it has to bootstrap build tooling on a node (i.e.
  `nix` wasn't already on its `PATH`), it also runs
  `scripts/proxmox/configure-nix-cache-client.sh` there (non-fatally — a
  failure just falls back to building from source / `cache.nixos.org`) so
  the node substitutes from and can offload builds to nix-cache on every
  subsequent run, not just this one.
- `scripts/proxmox/configure-nix-cache-client.sh [--dry-run]
  [--no-remote-builder] [--no-restart]` — the non-NixOS equivalent of
  `modules/nix-cache/client.nix`/`remote-builder-client.nix`, for a plain
  Debian machine with the Nix package manager (not NixOS) already
  installed: run as root *on that machine* to add nix-cache as a
  substituter in `/etc/nix/nix.conf` (`https://cache.nixos.org/` kept as
  fallback) via `extra-substituters`/`extra-trusted-public-keys` so it
  layers on top of whatever's already there instead of clobbering it, and,
  if `/root/.ssh/nixremote` is already present (see docs/nix-cache.md
  "Remote builder SSH keys"), configures it as a distributed-build
  machine too and trusts nix-cache's SSH host key in
  `/etc/ssh/ssh_known_hosts`. Idempotent (re-running replaces its own
  marked block rather than duplicating it); restarts `nix-daemon` by
  default so the change takes effect immediately.

### `scripts/lib/`

Sourced by the scripts above, never run directly:

- `nix-bootstrap.sh` — `NIX_CONFIG`/`ensure_nix_profile`, shared by
  `codex-setup.sh`/`codex-maintenance.sh` and the remote build commands
  `create-proxmox-resource.sh` runs over SSH.
- `nix-eval.sh` — `NIX_EVAL_FLAGS` plus `list_flake_targets`/
  `flake_target_hostname` flake-introspection helpers.
- `ssh-host-keys.sh` — `generate_host_ed25519_key`/`ssh_pubkey_to_age`,
  shared by `sync-host-keys.sh` and `prepare-host-key.sh`.
- `sops-age.sh` — `age_pubkey_from_identity_file`/`sops_yaml_admin_pubkey`/
  `sops_updatekeys` plus the shared sops/age default key-file resolution,
  shared by `backup-admin-key.sh`, `rotate-admin-key.sh`, and
  `sync-host-keys.sh`.
- `confirm.sh` — `confirm_typed`, the "type X back to confirm" destructive-
  action prompt shared by `create-proxmox-resource.sh` and
  `sync-host-keys.sh`.
- `sync-host-keys-edit-sops.py` — the `.sops.yaml` anchor/key_groups editor
  `sync-host-keys.sh` shells out to (see that script for why: precise,
  idempotent YAML edits are impractical in bash).

### Top level

- `scripts/env.sh` — shared config (`PROXMOX_HOST`, storage pool, bridge,
  default cores/memory, `NIX_CACHE_HOST`, `LAN_DOMAIN`) sourced by
  `create-proxmox-resource.sh` and `scripts/installer/auto-install.sh`. Add
  new cross-script config here instead of duplicating it per-script.
- `scripts/bump-nixpkgs-release.sh` — bumps `flake.nix`'s `nixpkgs.url`/
  `home-manager.url` in place. Exists because flake input URLs can't
  reference `variables.nix` (confirmed empirically — `nix flake metadata`
  errors on it), so this is the closest equivalent to a single source of
  truth for the tracked release.

`sync-host-keys.sh`, `create-proxmox-resource.sh`, and
`rotate-admin-key.sh` genuinely mutate real state when run for real (not
`--dry-run`): real `secrets/*.yaml` recipients, real Proxmox VMs/
containers, real revocation of decrypt access. They require the
operator's own SSH/sops access, which an agent session doesn't have — but
don't suggest running any of them non-dry-run without the operator's
explicit go-ahead even if it becomes technically reachable.
`backup-admin-key.sh` only writes a key copy to a path the operator gives
it — lower-stakes than the others, but it still handles a real private
key, so treat its destination path choice as the operator's call too.

## Architecture

`flake.nix` is the single entry point. It generates one
`nixosConfigurations.<platform>-<buildtype>` attribute per target via the
`mkTarget` function, composed from:

```
nixosSystem {
  modules = [
    disko.nixosModules.disko
    sops-nix.nixosModules.sops
    ./modules/common/configuration.nix
    ./modules/platforms/${platform}.nix        # what it runs on
    ./modules/build-types/${buildType}.nix     # what it's for
    hostPath                                   # hosts/<name>/host.nix — per-machine identity
    home-manager.nixosModules.home-manager { ... }
  ] ++ (client-only modules, for every buildType except "nix-cache" itself)
}
```

Platforms: `linode`, `proxmox`, `lxc`, `baremetal`. Build types: `minimal`,
`nix-cache`, `server`, `docker`, `gui`, `pxe-boot`, `tailscale-exit-node`,
`tor-relay`. Not every combination is built — e.g. `pxe-boot` has no `linode`
variant (PXE/DHCP/TFTP need LAN L2 adjacency a Linode VPS doesn't have),
`tor-relay` currently only exists as `lxc-tor-relay`, and `baremetal`
currently only exists as `baremetal-gui` (the real gui-host hardware —
see `hosts/nixos/host.nix` and `modules/platforms/baremetal.nix`). Treat
`flake.nix`'s
`generatedTargets` as the source
of truth for which hosts exist — `README.md`, `AGENTS.md`,
`docs/flake-lock-automation.md`, and the CI eval workflows
(`.github/workflows/check-nixos.yml`, `.gitea/workflows/check-nixos.yml`) list
hosts by hand (or, for the CI workflows, evaluate the flake dynamically) and
can drift from it, so re-check them against `flake.nix` when adding or
removing a host.

### Composition pattern

- `hosts/<name>/host.nix` — per-machine identity **only**: hostname, hostId,
  per-machine secrets, `system.stateVersion`. These files carry no `imports`
  of their own beyond narrow parameterized helpers (see
  `modules/beszel/host-token.nix` below) — all shared behavior comes from the
  platform/build-type modules composed in `flake.nix`, not from the host file.
- `modules/platforms/{linode,proxmox,lxc,baremetal}.nix` — platform-specific
  config: boot method, guest tooling, and the hardware config, imported
  directly by the platform module itself — **not** wired in from
  `flake.nix`. VM platforms use `../hardware-configuration/vm/{proxmox,linode}.nix`;
  `baremetal.nix` uses `../hardware-configuration/baremetal.nix` (adapted
  from a real `nixos-generate-config` run on the actual hardware, not a
  vm/ file, since it isn't a VM) plus `hardware.enableRedistributableFirmware
  = true` for real wifi/GPU/microcode firmware that VMs never needed.
  `lxc.nix` has no hardware-configuration counterpart since containers
  share the host kernel; instead it imports nixpkgs' own
  `virtualisation/proxmox-lxc.nix`, which gives every `lxc-*` host a
  `config.system.build.tarball` output — a plain rootfs tarball, used as a
  `pct create ... vztmpl` CT template (**not** `pct restore`, which expects
  `vzdump` backup-archive metadata this doesn't have), no install step —
  see `docs/auto-installer.md`.
- `modules/build-types/*.nix` — what a system is for:
  minimal/server/docker/gui/pxe-boot/nix-cache/tailscale-exit-node/tor-relay.
- `modules/common/configuration.nix` — base NixOS config imported by every
  host: locale, users, nix settings, git.
- `modules/common/home.nix` / `hosts/nixos/home.nix` — Home Manager config for
  the `nixos` user; the `nixos` workstation (`gui` build type) has its own,
  other hosts share `modules/common/home.nix`.
- `modules/disko/proxmox.nix` — declarative disk layout (GPT: ESP + swap +
  ext4 root) via disko, used by all Proxmox-VM hosts (`proxmox-*`, not
  `lxc-*`). Also carries `imageSize`/`imageName`, letting every `proxmox-*`
  host be built as a standalone, `qm importdisk`-ready `.raw` image with no
  install step — see `docs/proxmox-images.md`.
- `modules/disko/linode.nix` — `linode-*`'s disko config, deliberately
  different in kind from the Proxmox one: Linode provisions and sizes
  `/dev/sda`/`/dev/sdb` itself as whole, unpartitioned devices before the OS
  boots, so this declares them with `destroy = false` (disko never wipes
  them) and a bare `filesystem`/`swap` content type instead of a partition
  table — idempotent against an already-provisioned disk, never destructive.
- `modules/disko/baremetal.nix` — `baremetal-gui`'s disko config: a ZFS
  RAID0 (striped, no redundancy — disko's zpool `mode` defaults to `""`,
  which is a plain stripe rather than `"mirror"`/`"raidz"`) root pool
  across two disks, ESP + systemd-boot on the first. Device paths
  (`vars.guiRootDisk1`/`guiRootDisk2`) are placeholders — fill in stable
  `/dev/disk/by-id/...` paths before running disko for real.
  `modules/platforms/baremetal.nix` also imports
  `modules/services/zfs/enable-service.nix` for this (the `zfs_unstable`
  package, autoScrub/autoSnapshot/trim) — the only other importer today is
  `server`'s NFS data pool, an unrelated non-root ZFS use.
- `modules/boot/efi.nix` — systemd-boot + EFI vars, paired with the disko module.
- `modules/installer/` — the auto-installer environment (ISO, also served as
  PXE netboot): `common.nix` (shared config + the generated
  `auto-install.sh`), `iso.nix`, `host-keys.nix` (optionally bakes
  `host-keys/` into the image under `--impure`). See
  `docs/auto-installer.md`.
- `modules/pxe-boot/stage-installer-artifacts.nix` — builds the installer's
  netboot image and stages it on the `pxe-boot` host so its iPXE menu can
  chain straight to it. See `docs/pxe-boot.md`.
- `modules/nix-cache/{client,server,remote-builder-client}.nix` — binary cache
  substituter + SSH remote-builder wiring; see `docs/nix-cache.md` for the
  full design (per-host local stores, no shared `/nix/store`, and how the
  `nixremote` signing/SSH keys fit together).
- `modules/beszel/host-token.nix` — parameterized helper module
  (`{ name, sopsFile }`) that wires a host's beszel-agent sops secret/template
  and `environmentFile`; used by `hosts/server/host.nix` and
  `hosts/nix-cache/host.nix` to avoid duplicating that boilerplate.
- `modules/tailscale/`, `modules/docker/`, `modules/networking/`,
  `modules/traefik/`, `modules/tor/`, `modules/services/*` — single-purpose,
  single-host
  feature modules (e.g. `docker/enable-service.nix`,
  `services/zfs/enable-service.nix`). Grep `modules/build-types/*.nix` for
  each build type's `imports` list to see which modules apply where.

New host = new `hosts/<name>/host.nix` + a matching
`mkTarget { platform; buildType; hostPath; }` entry added to `flake.nix`'s
`generatedTargets`, composed from existing `modules/*` pieces rather than
duplicating config.

### Other docs worth reading before touching these areas

- `docs/nix-cache.md` — nix-cache binary cache/remote-builder design and key
  handling.
- `docs/pxe-boot.md` — the `pxe-boot` host's iPXE/TFTP/HTTP boot chain and
  directory layout under `/srv/pxe`.
- `docs/auto-installer.md` — the installer environment (ISO/netboot/Proxmox
  LXC), `host-keys/` and the sops-nix pre-seeding problem it solves, and why
  `lxc-*` hosts are deliberately excluded from its menu.
- `docs/proxmox-images.md` — building `proxmox-*` hosts as standalone `.raw`
  disk images (disko's image builder) instead of installing, and deploying
  the result to Proxmox.
- `docs/flake-lock-automation.md` — how `flake.lock` updates flow through CI
  (scheduled `nix flake update` PR + host-eval-on-PR workflow) and why hosts
  should track the committed lock file rather than `nixos-rebuild --upgrade-all`.