refactor: reorganize directory structure (variants -> hw, glue -> fleet)

- Rename variants/ to hw/ for clearer hardware module naming
- Rename glue/ to fleet/ for more intuitive fleet management
- Move boot/fs configuration from glue/boot.nix to separate fleet/boot.nix and fleet/fs.nix
- Improve separation of concerns between boot, filesystem, and common config

fleet/common.nix

@@ -0,0 +1,51 @@
+# ============================================================================
+# 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 = [
+    ./fs.nix
+    ./boot.nix
+    ./user-config.nix
+    ../sw
+    ../users.nix
+  ];
+
+  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.
+      '';
+    };
+    host.useHostPrefix = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = "Whether to prepend the host prefix to the hostname (used in inventory and hosts/default.nix).";
+    };
+  };
+
+  config = lib.mkMerge [ 
+    (lib.mkIf (config.athenix.forUser != null) {
+      athenix.users.${config.athenix.forUser}.enable = true;
+    }) 
+    { 
+      system.stateVersion = "25.11"; 
+      nix.settings.experimental-features = [
+        "nix-command"
+        "flakes"
+      ];
+    }
+  ];
+}