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/docker/enable-service.nix

@@ -1,19 +1,9 @@
 { pkgs, vars, ... }:
 
 {
-  #  virtualisation.docker.enable = true;
   virtualisation.docker = {
     enable = true;
     package = pkgs.docker;
-    #  listenOptions = [
-    #  "unix:///var/run/docker.sock"
-    #  "tcp://0.0.0.0:2375"
-    #];
-
-    #  daemon.settings = {
-    #    metrics-addr = "0.0.0.0:9323";
-    #    experimental = true;
-    #  };
   };
   users.users.${vars.primaryUser}.extraGroups = [ "docker" ];
   environment.systemPackages = with pkgs; [