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/en.json

@@ -302,6 +302,8 @@
     "loadingSkill": "Loading skill...",
     "emptyNoAgents": "No available agent.",
     "noSelectionHint": "Select a skill on the left, or click \"New Skill\" to create one.",
+    "systemBadge": "System",
+    "systemHint": "Built-in CLI skill · read-only",
     "scope": {
       "global": "Global",
       "folder": "Folder",