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/xdg/desktop-entries.nix

@@ -46,7 +46,7 @@
       displayName = "ChatGPT";
       url = "https://chatgpt.com/";
       iconUrl = "https://static.vecteezy.com/system/resources/previews/031/110/149/large_2x/chatgpt-logo-transparent-free-png.png";
-      iconSha = "sha256-ZWmhchblQkksW02eduVrkUSPAlWPGC2fjqxrAGAF5jw=";
+      iconSha = "sha256-WXR/d28ynaBe+kZpnkUAaHcriDVSge98weS5Dza+AvQ=";
     }
     {
       name = "claude";