feat(codex-skills): surface built-in .system skills as read-only

Scan ~/.codex/skills/.system so Codex CLI's bundled skills (imagegen,
skill-creator, etc.) appear in the $ autocomplete and Skills settings.
Mark them read_only on the API; the save/delete handlers refuse writes
to that path, and the UI shows a system badge with a tooltip and
disables edit/delete/save for those entries.

src/i18n/messages/ja.json

@@ -302,6 +302,8 @@
     "loadingSkill": "Skillを読み込み中...",
     "emptyNoAgents": "利用可能なエージェントがありません。",
     "noSelectionHint": "左側から Skill を選択するか、「新規 Skill」をクリックして作成してください。",
+    "systemBadge": "システム",
+    "systemHint": "CLI 組み込み Skill・読み取り専用",
     "scope": {
       "global": "グローバル",
       "folder": "フォルダ",