dns: auto-generate zone entries from host configurations

Replace static zone file with dynamically generated records:
- Add homelab.dns module with enable/cnames options
- Extract IPs from systemd.network configs (filters VPN interfaces)
- Use git commit timestamp as zone serial number
- Move external hosts to separate external-hosts.nix

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hosts/nix-cache01/configuration.nix

@@ -11,6 +11,8 @@
     ../../common/vm
   ];
 
+  homelab.dns.cnames = [ "nix-cache" "actions1" ];
+
   fileSystems."/nix" = {
     device = "/dev/disk/by-label/nixcache";
     fsType = "xfs";