Enable ChatGPT and restructure Codex module

- Set `chatgpt.enable = true` for both Garand‑Desktop and Garand‑Laptop.
- Rename `modules/home/codex.nix` to `modules/home/codex/default.nix`.
- Add `mutableSettings = true` and import `plugins.nix` in the Codex
  module.
- Introduce `modules/home/codex/plugins.nix` defining Codex plugins and
  marketplaces.
- Update `modules/home/default.nix` to import the new Codex directory.
- Refresh ChatGPT desktop entry icon SHA256 hash.

modules/home/codex/default.nix

@@ -0,0 +1,18 @@
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}: {
+  options.codex.enable = lib.mkEnableOption "Codex";
+
+  config.programs.codex = lib.mkIf config.codex.enable {
+    enable = true;
+    package = pkgs.codex;
+    mutableSettings = true;
+  };
+
+  imports = [
+    ./plugins.nix
+  ];
+}