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

hw/default.nix

@@ -0,0 +1,28 @@
+# ============================================================================
+# Host Types Module
+# ============================================================================
+# This module exports all available host types as an attribute set.
+# Each type is a NixOS module (a function suitable for lib.types.submodule).
+
+{ inputs }:
+let
+  lib = inputs.nixpkgs.lib;
+  inherit (builtins) readDir attrNames;
+  inherit (lib) filterAttrs removeSuffix genAttrs;
+
+  files = readDir ./.;
+
+  # Keep only regular *.nix files except default.nix
+  nixFiles = filterAttrs (
+    name: type:
+      type == "regular"
+      && lib.hasSuffix ".nix" name
+      && name != "default.nix"
+  ) files;
+
+  moduleNames = map (name: removeSuffix ".nix" name) (attrNames nixFiles);
+in
+# Export: { name = <module function from ./name.nix>; }
+genAttrs moduleNames (name:
+  import (./. + ("/" + name + ".nix"))
+)