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>

docs/auto-installer.md

@@ -22,7 +22,7 @@ see "LXC hosts" immediately below for why those are different.**
 ## LXC hosts
 
 `lxc-*` targets (`lxc-minimal`, `lxc-nix-cache`, `lxc-server`, `lxc-docker`,
-`lxc-gui`, `lxc-pxe-boot`, `lxc-tailscale-exit-node`, `lxc-tor-relay`) are **not** installed via `auto-install.sh` — the
+`lxc-gui`, `lxc-pxe-boot`, `lxc-tailscale-router`, `lxc-tor-relay`) are **not** installed via `auto-install.sh` — the
 interactive menu deliberately excludes them. Don't try to select one there;
 `nixos-install` would bind-mount `/` onto `/mnt` (LXC containers have no raw
 disk to partition) and then refuse to touch the filesystem it's currently
@@ -133,13 +133,15 @@ Flake outputs:
 ```nix
 nixosConfigurations.installer   # ISO/netboot installer image
 
-packages.x86_64-linux.iso   # installer ISO/netboot image
-packages.x86_64-linux.pxe   # netboot-ipxe + netboot-initrd + netboot-kernel, bundled
+packages.x86_64-linux.iso       # installer ISO/netboot image
+packages.x86_64-linux.pxe       # auto-installer netboot bundle (kernel + initrd + ipxe script)
+packages.x86_64-linux.pxe-minimal  # vanilla NixOS minimal netboot bundle (no installer wiring)
 ```
 
 ```sh
 nix build .#iso
 nix build .#pxe
+nix build .#pxe-minimal
 ```
 
 There's no `nixosConfigurations.proxmox-lxc` (installer-boots-as-an-LXC-