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/lib.rs

@@ -376,6 +376,7 @@ mod tauri_app {
                 acp_commands::acp_save_agent_skill,
                 acp_commands::acp_delete_agent_skill,
                 experts_commands::experts_list,
+                experts_commands::experts_list_for_agent,
                 experts_commands::experts_get_install_status,
                 experts_commands::experts_link_to_agent,
                 experts_commands::experts_unlink_from_agent,