Migrate host-key tooling from nix-auto-installer, bake keys into images

Finishes the nix-auto-installer migration: scripts/prepare-host-key.sh
and the local host-keys/ directory (gitignored, private key material,
never committed — moved as plain files, not through git history)
weren't carried over in the initial migration.

Also implements automatic key staging, replacing the manual
scp-after-boot step:

- modules/installer/host-keys.nix reads host-keys/ via
  builtins.getEnv, which Nix silently returns as "" under normal
  (non---impure) evaluation — the module is a no-op by default, safe
  for CI, until explicitly opted into:

    NIXOS_HOST_KEYS_DIR=$(pwd)/host-keys nix build .#iso --impure

  When built this way every key present gets baked into the image at
  /etc/host-keys/, and auto-install.sh installs whichever one matches
  the flake target selected at install time — no manual per-host scp.

- This deliberately includes the PXE netboot variant, even though
  pxe-boot serves it unauthenticated over LAN HTTP: accepted
  explicitly as a reasonable trade-off for a network that sits behind
  LAN-only infrastructure, not the open internet. auto-install.sh
  still falls back to /root/host-keys (manual scp) if a key isn't
  baked in, so images built without --impure keep working exactly as
  before.

- docs/auto-installer.md replaces nix-auto-installer's README,
  updated for in-repo paths and the new build flow.

Verified: normal `nix eval` (no --impure) evaluates identically across
all 19 nixosConfigurations + 4 packages with zero host-keys/* entries
(CI-unaffected); with --impure + the env var set, all three installer
variants (installer/ISO, proxmox-lxc, pxe) correctly embed every key
in host-keys/.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

scripts/prepare-host-key.sh

@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+# Generates a new machine's SSH host key ahead of installing it, so
+# sops-nix has something to derive an age key from before the target
+# ever boots.
+#
+# Why this is needed: sops-nix derives each host's decryption key from
+# its own /etc/ssh/ssh_host_ed25519_key at *activation* time, but that
+# activation runs before systemd would otherwise generate this key on
+# first boot (sshd-keygen is a normal systemd service gated behind
+# multi-user.target; activation scripts run earlier than that). Without
+# pre-seeding, secrets — including the root/nixos login password — fail
+# to decrypt on the machine's very first boot.
+#
+# This script only touches your admin workstation and this repo's
+# .sops.yaml (it never contacts the target machine). Run it, follow the
+# printed next steps, then use the resulting key with the auto-install.sh
+# prompt (see modules/installer/common.nix) when you actually install the
+# new machine.
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "$0")/.." && pwd)"
+
+hostname="${1:?usage: scripts/prepare-host-key.sh <hostname> [path-to-nixos-repo]}"
+nixos_repo="${2:-$repo_root}"
+sops_yaml="${nixos_repo}/.sops.yaml"
+
+if [[ ! -f "$sops_yaml" ]]; then
+  echo "ERROR: $sops_yaml not found. Pass the nixos repo path as the 2nd argument." >&2
+  exit 1
+fi
+
+keydir="${repo_root}/host-keys"
+mkdir -p "$keydir"
+keyfile="${keydir}/${hostname}_ssh_host_ed25519_key"
+
+if [[ -f "$keyfile" ]]; then
+  echo "ERROR: $keyfile already exists. Remove it first if you want to regenerate." >&2
+  exit 1
+fi
+
+nix-shell -p openssh --run "ssh-keygen -t ed25519 -N '' -C '${hostname}' -f '${keyfile}'" >/dev/null
+
+age_pub="$(nix-shell -p ssh-to-age --run "ssh-to-age -i '${keyfile}.pub'")"
+
+cat <<EOF
+
+Generated: ${keyfile}(.pub)
+
+=== 1. Add this line under keys: in ${sops_yaml} ===
+  - &${hostname} ${age_pub}
+
+=== 2. Add *${hostname} to whichever creation_rules key_groups this host needs ===
+(e.g. secrets/common.yaml always; add a per-host secrets/${hostname}.yaml
+ block too if this host will get its own secrets, same pattern as
+ nix-cache/server.)
+
+=== 3. Re-encrypt every secrets file you just added it to ===
+  nix-shell -p sops --run 'sops updatekeys ${nixos_repo}/secrets/common.yaml'
+
+=== 4. Commit + push this repo so the flake build picks up the new recipient ===
+
+=== 5. Get the key onto the installer, one of two ways ===
+  a) Rebuild the installer image with all host-keys/ baked in (see
+     docs/auto-installer.md):
+       NIXOS_HOST_KEYS_DIR="${keydir}" nix build .#iso --impure
+     (or .#lxc / .#pxe / .#all — --impure is required since host-keys/
+     is gitignored and flakes can't see it otherwise)
+
+  b) Or, for an image already built without keys, scp it in after boot:
+       scp ${keyfile}{,.pub} root@<target-ip>:/root/host-keys/
+
+Then continue with /etc/auto-install.sh as normal — it checks
+/etc/host-keys (baked in) before /root/host-keys (scp'd) and installs
+whichever it finds before running nixos-install.
+EOF