refactor: use flake package directly in NixOS module

Instead of requiring users to provide the package via overlay,
the module now receives `self` from the flake and uses the
package directly from `self.packages`.

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

flake.nix

@@ -49,7 +49,7 @@
           };
         });
 
-      nixosModules.default = import ./nixos/module.nix;
+      nixosModules.default = import ./nixos/module.nix { inherit self; };
       nixosModules.homelab-deploy = self.nixosModules.default;
     };
 }