feat(#2038): surface degraded mcp servers at turn start (observability)

hive-agent/src/turn.rs

@@ -628,6 +628,7 @@ impl Sink for BusSink<'_> {
         // and is applied from the run's returned `Telemetry` (see `drive_turn`
         // → `apply_telemetry`).
         self.bus.observe_stream(event);
+        self.bus.observe_mcp_health(event);
         self.bus.emit(LiveEvent::Stream(event.clone()));
     }