refactor: move GC and buildMethods options to sw module

- Move athenix.system.gc options from fleet/common.nix to sw/gc.nix
- Move athenix.host.buildMethods option to sw/gc.nix
- Move home-manager, agenix, disko imports to sw/default.nix
- Better separation: fleet/ handles generation, sw/ handles software config

sw/default.nix

@@ -28,8 +28,12 @@ in
   imports = [
     ./python.nix
     ./ghostty.nix
+    ./gc.nix
     ./updater.nix
     ./update-ref.nix
+    inputs.home-manager.nixosModules.home-manager
+    inputs.agenix.nixosModules.default
+    inputs.disko.nixosModules.disko
   ];
 
   options.athenix.sw = {