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 = {

sw/gc.nix

@@ -0,0 +1,58 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+{
+  options.athenix = {
+    system.gc = {
+      enable = lib.mkOption {
+        type = lib.types.bool;
+        default = true;
+        description = "Whether to enable automatic garbage collection.";
+      };
+      frequency = lib.mkOption {
+        type = lib.types.str;
+        default = "weekly";
+        description = "How often to run garbage collection (systemd timer format).";
+      };
+      retentionDays = lib.mkOption {
+        type = lib.types.int;
+        default = 30;
+        description = "Number of days to keep old generations before deletion.";
+      };
+      optimise = lib.mkOption {
+        type = lib.types.bool;
+        default = true;
+        description = "Whether to automatically optimize the Nix store.";
+      };
+    };
+
+    host.buildMethods = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [ "installer-iso" ];
+      description = ''
+        List of allowed build methods for this host (used by installer/artifacts.nix).
+        Supported methods:
+        - "installer-iso": Generates an auto-install ISO that installs this configuration to disk.
+        - "iso": Generates a live ISO (using nixos-generators).
+        - "ipxe": Generates iPXE netboot artifacts (kernel, initrd, script).
+        - "lxc": Generates an LXC container tarball.
+        - "proxmox": Generates a Proxmox VMA archive.
+      '';
+    };
+  };
+
+  config = {
+    # Automatic Garbage Collection
+    nix.gc = lib.mkIf config.athenix.system.gc.enable {
+      automatic = true;
+      dates = config.athenix.system.gc.frequency;
+      options = "--delete-older-than ${toString config.athenix.system.gc.retentionDays}d";
+    };
+
+    # Optimize storage
+    nix.optimise.automatic = config.athenix.system.gc.optimise;
+  };
+}