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/pt.json

@@ -302,6 +302,8 @@
     "loadingSkill": "Carregando Skill...",
     "emptyNoAgents": "Nenhum agente disponível.",
     "noSelectionHint": "Selecione um Skill à esquerda ou clique em \"Novo Skill\" para criar um.",
+    "systemBadge": "Sistema",
+    "systemHint": "Skill integrado do CLI · somente leitura",
     "scope": {
       "global": "Global",
       "folder": "Pasta",