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>

AGENTS.md

@@ -6,12 +6,14 @@ This repository contains flake-based NixOS configurations for Wayne's LAN
 servers and workstation.
 
 The flake exposes NixOS configurations named `<platform>-<buildtype>`
-(platforms: `linode`, `proxmox`, `lxc`; build types: `minimal`, `nix-cache`,
-`server`, `docker`, `gui`, `pxe-boot`, `tailscale-exit-node`, `tor-relay`), generated from `modules/platforms/*`
-and `modules/build-types/*` by the `mkTarget` function in `flake.nix`. Not
-every combination is built — `pxe-boot` has no `linode` variant. See
-`README.md` for the full current target list; treat `flake.nix` as the
-source of truth since this list can drift.
+(platforms: `linode`, `proxmox`, `lxc`, `baremetal`; build types: `minimal`,
+`nix-cache`, `server`, `docker`, `gui`, `pxe-boot`, `tailscale-router`,
+`tor-relay`, `ha-server`), generated from `modules/platforms/*` and
+`modules/build-types/*` by the `mkTarget` function in `flake.nix`. Not every
+combination is built — `pxe-boot` has no `linode` variant, `ha-server` only
+exists on `proxmox`, and `tor-relay` only exists on `lxc`. See `README.md`
+for the full current target list; treat `flake.nix` as the source of truth
+since this list can drift.
 
 Do not deploy, switch, reboot, repartition, format disks, or run destructive
 install commands from this repository unless explicitly asked.