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>

modules/common/configuration.nix

@@ -25,13 +25,10 @@ let
   '';
 in
 {
-  imports =
-    [
-      # Include the results of the hardware scan.
-      #      ./hardware-configuration.nix
-      ./set-locale.nix
-      ../ipa/client.nix
-    ];
+  imports = [
+    ./set-locale.nix
+    ../ipa/client.nix
+  ];
 
   # System-wide shell config so all users (including IPA accounts) get the
   # same management aliases as the local nixos user's Home Manager provides.
@@ -42,11 +39,7 @@ in
     };
     interactiveShellInit = buildImageFn;
   };
-  # Use the GRUB 2 boot loader.
-  # boot.loader.grub.enable = true;
-  #boot.loader.grub.device = "/dev/sda"; # or "nodev" for efi only
-
-  networking.networkmanager.enable = true; # Easiest to use and most distros use this by default.
+  networking.networkmanager.enable = true;
 
   # Recommended over the true default (bypasses ZFS's own import safeguards)
   # per the option's own docs; matches hosts/docker/host.nix and