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

variants/default.nix

@@ -0,0 +1,29 @@
+# ============================================================================
+# Host Types Module
+# ============================================================================
+# This module exports all available host types as an attribute set.
+# Each type is a NixOS module function that takes { inputs } and returns
+# a module configuration.
+
+{ inputs }:
+let
+  inherit (builtins) readDir attrNames;
+  lib = inputs.nixpkgs.lib;
+  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
+genAttrs moduleNames
+  (name:
+    import ./${name}.nix { inherit inputs; })