feat(runtime): track node lifecycle and QUIC link state

backend/agent/src/MoShiro.Agent/AgentApp.cs

@@ -282,6 +282,7 @@ public sealed class AgentApp : IDisposable
             Capabilities = [MoShiroCapabilities.Base, MoShiroCapabilities.KindsPrefix + string.Join(',', BlockCatalog.AgentKinds)],
             HelloMeta = HelloMeta,
             HeartbeatExtra = HeartbeatExtra,
+            ShutdownReason = () => _restart.NowReason == "update" ? "updating" : "stopping",
         };
         var link = new FleetLink(options, router, [new StateHandler<TaskSet>(TaskSet.StateType, MoShiroJsonContext.Default.TaskSet, _tasks.ApplyAsync)], _outbox, _time, _log);
 
@@ -335,6 +336,7 @@ public sealed class AgentApp : IDisposable
         long packetsOut = _counters.PacketsOut;
         var extra = new AgentHeartbeatExtra
         {
+            RuntimeStatus = _update?.Status().State == "applying" ? NodeRuntimeStatus.Updating : NodeRuntimeStatus.Running,
             Ipv6 = Ipv6Diagnostics.Sample(_ipv6Supported, Environment.GetEnvironmentVariable("MOSHIRO_NETWORK_SCOPE"), Environment.GetEnvironmentVariable("HOST_PROC")),
             Probes = [.. _hub.ConnectedProbes()],
             Sessions = _sessions.All.Count,