refactor(ipa): move FreeIPA client to common config with auto-enrollment

modules/ipa/client.nix is now a self-contained NixOS module (no longer a
parameterized function): it checks builtins.pathExists for
secrets/<hostname>.keytab at eval time and enables itself automatically if
found, making it a no-op for hosts without a keytab.

modules/common/configuration.nix imports it so every host in the flake is
a candidate for IPA enrollment — no per-host wiring needed. Adding a
keytab (via scripts/ipa/create-nixos-ipa-host-account.sh) is now the
only step required to enroll a host.

The module also sets networking.domain and networking.nameservers via
mkDefault when active, so new hosts don't need those set explicitly.

Also:
- Remove explicit IPA imports from hosts/nix-cache and hosts/tailscale-router
- Add secrets/pxe-boot.keytab + creation rule; remove incorrect
  secrets/nixos.sweet.home.keytab and its creation rule
- Add .sops.yaml creation rules for all remaining host keytabs
  (server, docker, tor-relay, nix-minimal, nixos) so the creation script
  can target them without manual .sops.yaml edits
- Fix duplicate tailscale-router.keytab rule and corrupted gui.yaml comment
  block in .sops.yaml

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

hosts/nix-cache/host.nix

@@ -6,15 +6,10 @@
       name = "nix-cache";
       sopsFile = ../../secrets/nix-cache.yaml;
     })
-    (import ../../modules/ipa/client.nix {
-      keytabSopsFile = ../../secrets/nix-cache.keytab;
-      caCertFile = ../../certs/ipa-ca.crt;
-    })
   ];
 
   networking = {
     hostName = vars.nixCacheHost;
-    domain = vars.homeDomain;
     useDHCP = false;
     interfaces.${vars.lxcLanInterface}.ipv4.addresses = [{
       address = vars.nixCacheIp;