refactor: create glue/ and variants/ directories

- Add glue/ for fleet generation logic and common configuration
- Add variants/ for hardware type modules
- Improves separation of concerns and module organization

glue/common.nix

@@ -0,0 +1,99 @@
+# ============================================================================
+# Common Host Module
+# ============================================================================
+# This module contains all the common configuration shared by all host types.
+# It is automatically imported by the fleet generator for every host.
+
+{ inputs }:
+{
+  config,
+  lib,
+  ...
+}:
+{
+  imports = [
+    ./boot.nix
+    ./user-config.nix
+    ../sw
+    ../users.nix
+    inputs.home-manager.nixosModules.home-manager
+    inputs.agenix.nixosModules.default
+    inputs.disko.nixosModules.disko
+  ];
+
+  # Define garbage collection options here since they're consumed in this module
+  options.athenix = {
+    forUser = lib.mkOption {
+      type = lib.types.nullOr lib.types.str;
+      default = null;
+      description = ''
+        Convenience option to configure a host for a specific user.
+        Automatically enables the user (sets athenix.users.username.enable = true).
+        Value should be a username from athenix.users.accounts.
+      '';
+    };
+
+    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 = lib.mkMerge [
+    # Enable forUser if specified
+    (lib.mkIf (config.athenix.forUser != null) {
+      athenix.users.${config.athenix.forUser}.enable = true;
+    })
+
+    {
+      system.stateVersion = "25.11";
+
+      nix.settings.experimental-features = [
+        "nix-command"
+        "flakes"
+      ];
+
+      # 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;
+    }
+  ];
+}