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/plugins.nix

@@ -0,0 +1,48 @@
+{pkgs, ...}: let
+  ponytail = pkgs.fetchFromGitHub {
+    name = "ponytail";
+    owner = "DietrichGebert";
+    repo = "ponytail";
+    tag = "v5.3.2";
+    sha256 = "sha256-IJlhfkdXI6pMZZCZO7XBRg88LBiNfzc5Gnm4ofGybL8=";
+  };
+
+  context-mode = pkgs.fetchFromGitHub {
+    name = "context-mode";
+    owner = "mksglu";
+    repo = "context-mode";
+    tag = "v1.0.169";
+    sha256 = "sha256-1pV56ZB2aqod+C0kb5myuiWLAJ7+opiaurwZZ3BGKYk=";
+  };
+
+  understand-anything-src = pkgs.fetchFromGitHub {
+    name = "understand-anything";
+    owner = "Egonex-AI";
+    repo = "Understand-Anything";
+    tag = "v2.9.0";
+    sha256 = "sha256-nECfbp0vyFZRZX0Zg2s4ga/kYSCihJxYt9lxUHLGmFA=";
+  };
+
+  understand-anything =
+    pkgs.runCommand "understand-anything" {
+      version = "2.9.0";
+      nativeBuildInputs = [pkgs.jq];
+    } ''
+      mkdir -p "$out"
+      cp -r ${understand-anything-src}/understand-anything-plugin/. "$out/"
+      mkdir -p "$out/.codex-plugin"
+      jq '. + {skills: "./skills/", hooks: "./hooks/hooks.json"}' \
+        "$out/.claude-plugin/plugin.json" > "$out/.codex-plugin/plugin.json"
+      test -f "$out/skills/understand/SKILL.md"
+      test -f "$out/hooks/hooks.json"
+    '';
+in {
+  programs.codex = {
+    plugins = [ponytail understand-anything context-mode];
+    marketplaces = {
+      ponytail = ponytail;
+      Understand-Anything = understand-anything-src;
+      context-mode = context-mode;
+    };
+  };
+}