Function handle_agent_holding 

pub(super) async fn handle_agent_holding(
    __arg0: State<AppState>,
    __arg1: Query<HoldingQuery>,
) -> Response

GET /agentmux/agent/holding?uid=… — a LAN peer asking whether this host runs uid (spec §4.4, Phase 4). LAN-key readable, like the agent-name list: it discloses only whether an agent UID is live here, since when, and in which channel/version.