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.

hosts/Garand-Desktop/home-modules.nix

@@ -73,7 +73,7 @@ _: {
       messenger.enable = true; # Messenger: Facebook Messenger
       mastodon.enable = true; # Mastodon: a decentralized social network
       garandcloud.enable = true; # GarandCloud: my Nextcloud instance
-      chatgpt.enable = false; # ChatGPT: a large language model
+      chatgpt.enable = true; # ChatGPT: a large language model
       claude.enable = true; # Claude: a large language model
       glance.enable = true; # Glance: my home server dashboard
       jellyfinClient.enable = true; # Jellyfin: My Jellyfin

hosts/Garand-Laptop/home-modules.nix

@@ -73,7 +73,7 @@ _: {
       messenger.enable = true; # Messenger: Facebook Messenger
       mastodon.enable = true; # Mastodon: a decentralized social network
       garandcloud.enable = true; # GarandCloud: my Nextcloud instance
-      chatgpt.enable = false; # ChatGPT: a large language model
+      chatgpt.enable = true; # ChatGPT: a large language model
       claude.enable = true; # Claude: a large language model
       glance.enable = true; # Glance: my home server dashboard
       jellyfinClient.enable = true; # Jellyfin: My Jellyfin