Remove k8s-server host

Drop k8s-server from flake, Makefile targets, modules
(kubernetes.nix, docker-registry.nix), install script and
hardware config. Remove related registry vhost, port 6443
and NAT rules from gateway.nix. Drop K8s-Server section
and dangling markdown wrapper from README.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flake.nix

@@ -24,13 +24,6 @@
           ./hosts/gateway/configuration.nix
         ];
       };
-
-      k8s-server = lib.nixosSystem {
-        inherit system;
-        modules = [
-          ./hosts/k8s-server/configuration.nix
-        ];
-      };
     };
   };
 }