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/default.nix

@@ -2,6 +2,7 @@ _: {
   imports = [
     ./bash
     ./chromium
+    ./codex
     ./fastfetch
     ./fish
     ./hyprland
@@ -19,7 +20,6 @@ _: {
     ./btop.nix
     ./cargo.nix
     ./cava.nix
-    ./codex.nix
     ./easyeffects.nix
     ./eza.nix
     ./fzf.nix