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>

nixos/module.nix

@@ -1,3 +1,4 @@
+{ self }:
 { config, lib, pkgs, ... }:
 
 let
@@ -21,7 +22,11 @@ in
   options.services.homelab-deploy.listener = {
     enable = lib.mkEnableOption "homelab-deploy listener service";
 
-    package = lib.mkPackageOption pkgs "homelab-deploy" { };
+    package = lib.mkOption {
+      type = lib.types.package;
+      default = self.packages.${pkgs.system}.homelab-deploy;
+      description = "The homelab-deploy package to use";
+    };
 
     hostname = lib.mkOption {
       type = lib.types.str;