chore: full sweep — docs sync, SSH key module extraction, NFS dedup, dead code removal

Documentation fixes:
- README/AGENTS: rename tailscale-exit-node → tailscale-router, add ha-server
  build type and proxmox-ha-server-{1,2} host table rows, add baremetal to
  platform list, remove references to non-existent flake-target-refactor-spec.md
  and remove-sensetive-info-refactor.md
- docs/auto-installer.md: fix lxc-tailscale-exit-node → lxc-tailscale-router,
  add pxe-minimal to the flake outputs list
- variables.nix: fix domainControllerIp comment — IPA is the authoritative DNS
  at .253 (Pi-hole is gone), not a forwarding intermediary

Code deduplication:
- Extract duplicate SSH host-key preservation activation scripts from
  modules/platforms/lxc.nix and modules/platforms/proxmox.nix into a shared
  modules/common/preserve-ssh-host-key.nix; both platforms now import it
- Replace 8-line hand-enumerated NFS export lists in server.nix and ha-server.nix
  with a mkNfsExports helper that generates exports from vars.nfsShares — adding
  a share to variables.nix now propagates to both exporters automatically

Dead code removal:
- modules/common/configuration.nix: remove leftover NixOS skeleton comments
  (hardware-configuration import, grub lines) that were never used
- modules/docker/enable-service.nix: remove commented-out listenOptions and
  daemon.settings blocks
- hosts/server/host.nix, hosts/nix-cache/host.nix: remove #DOCKER_HOST comments

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -10,13 +10,13 @@ pieces composed in `flake.nix`:
 
 - **Platforms** (what it runs on): `linode`, `proxmox`, `lxc`, `baremetal`
 - **Build types** (what it's for): `minimal`, `nix-cache`, `server`, `docker`,
-  `gui`, `pxe-boot`, `tailscale-exit-node`, `tor-relay`
+  `gui`, `pxe-boot`, `tailscale-router`, `tor-relay`, `ha-server`
 
 Not every combination exists — `pxe-boot` has no `linode` variant, since
 PXE/DHCP/TFTP need LAN L2 adjacency that 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). The
-full list:
+`tor-relay` and `ha-server` currently only exist on `lxc`/`proxmox`, and
+`baremetal` currently only exists as `baremetal-gui` (the real gui-host
+hardware). The full list:
 
 | Target | Purpose |
 | --- | --- |
@@ -29,8 +29,9 @@ full list:
 | `linode-gui` / `proxmox-gui` / `lxc-gui` | Cinnamon desktop workstation — previously the flat `nixos` target |
 | `baremetal-gui` | Same Cinnamon desktop workstation, on the real gui-host hardware — ZFS RAID0 root, systemd-boot |
 | `proxmox-pxe-boot` / `lxc-pxe-boot` | HTTP/iPXE boot asset host — previously the flat `pxe-boot` target |
-| `linode-tailscale-exit-node` / `proxmox-tailscale-exit-node` / `lxc-tailscale-exit-node` | Tailscale exit node |
+| `linode-tailscale-router` / `proxmox-tailscale-router` / `lxc-tailscale-router` | Tailscale subnet router + MagicDNS forwarder for the LAN |
 | `lxc-tor-relay` | Tor middle relay |
+| `proxmox-ha-server-1` / `proxmox-ha-server-2` | HA file-server cluster nodes — DRBD + XFS + iSCSI + NFS, managed by Corosync + Pacemaker |
 
 Which variant of a given buildtype is actually deployed isn't tracked
 anywhere in this repo — that's live infrastructure state, not something a
@@ -47,8 +48,7 @@ section for which is which.
 
 Each buildtype's `hosts/<name>/host.nix` carries the per-machine identity
 (hostname, hostId, per-machine secrets, `system.stateVersion`) that must stay
-fixed regardless of which platform it's built for — see
-`flake-target-refactor-spec.md` for the full rationale. Every deployed host
+fixed regardless of which platform it's built for. Every deployed host
 stamps its own active target name into `/etc/flake-target` at build time, so
 `nixos-rebuild switch --flake .#$(cat /etc/flake-target)` always picks up the
 right one even after a platform migration changes the flake attribute name.
@@ -167,7 +167,6 @@ per-boot host key for sops-nix to derive from on ephemeral media) — see
 (`vars/per-machine/<target>/openssh/`, committed and sops-encrypted) for
 their SSH host keys.
 
-This repository's git *history* still contains secrets committed before this
-migration (see `remove-sensetive-info-refactor.md`) — those are being
-scrubbed and rotated separately; don't treat the repo as safe to make public
-until that's finished.
+This repository's git *history* still contains secrets committed before the
+sops-nix migration — those are being scrubbed and rotated separately; don't
+treat the repo as safe to make public until that's finished.