fix(chat): query expert skills via symlinks and use $ prefix for Codex

Expert skills in the chat session were derived by intersecting built-in
experts with ACP availableCommands, which caused Codex experts to never
appear since Codex does not advertise skills through ACP.

- Add `experts_list_for_agent` backend API that checks symlink status
  across all global skill dirs for the given agent type
- Replace availableCommands-based expert filtering with symlink-based
  query, making the settings page the single source of truth
- Use `$` prefix for Codex expert skills while keeping `/` for slash
  commands and other agents' experts
- Disable the expert button when no experts are linked for the agent
- Invalidate per-agent expert cache after link/unlink in settings

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/web/router.rs

@@ -177,6 +177,7 @@ pub fn build_router(state: Arc<AppState>, token: String, static_dir: std::path::
         .route("/acp_delete_agent_skill", post(handlers::acp::acp_delete_agent_skill))
         // ─── Experts ───
         .route("/experts_list", post(handlers::experts::experts_list))
+        .route("/experts_list_for_agent", post(handlers::experts::experts_list_for_agent))
         .route("/experts_get_install_status", post(handlers::experts::experts_get_install_status))
         .route("/experts_link_to_agent", post(handlers::experts::experts_link_to_agent))
         .route("/experts_unlink_from_agent", post(handlers::experts::experts_unlink_from_agent))