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-tauri/src/acp/types.rs

@@ -302,6 +302,10 @@ pub struct AgentSkillItem {
     pub scope: AgentSkillScope,
     pub layout: AgentSkillLayout,
     pub path: String,
+    /// True for skills bundled by the agent CLI itself (e.g. Codex's
+    /// `~/.codex/skills/.system/*`). Surfaced so the UI can show them but
+    /// refuse to edit or delete; the backend also refuses such writes.
+    pub read_only: bool,
 }
 
 #[derive(Debug, Clone, Serialize)]