chore(modules): add auto-import default.nix files

modules/default.nix

@@ -0,0 +1,10 @@
+{ lib, ... }:
+
+{
+  imports =
+    let
+      dirs = lib.attrNames (lib.filterAttrs (_: v: v == "directory") (builtins.readDir ./.));
+      hasDef = name: builtins.pathExists ./${name}/default.nix;
+    in
+    map (name: ./${name}) (builtins.filter hasDef dirs);
+}

modules/desktops/default.nix

@@ -0,0 +1,8 @@
+{ ... }:
+
+{
+  imports = [
+    ./gnome
+    ./niri
+  ];
+}