feat: signed releases and a host-side updater for container nodes

CI signs the image digest of each agent and probe release; the panel only
stores and forwards it. Nodes write the manifest to their data directory and
a host updater verifies the signature with a local public key, pulls by
digest, restarts the unit and rolls back when it is unhealthy. The panel can
list releases, ask a node to update and show the reported update state.

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

@@ -25,6 +25,7 @@ public sealed class AgentApp : IDisposable
     private readonly TaskCompletionSource _ready = new(TaskCreationOptions.RunContinuationsAsynchronously);
     private readonly AgentAdmission _admission;
     private readonly Outbox _outbox = new(64 << 20);
+    private readonly UpdateBridge _update;
     private readonly BackendUplink _uplink;
     private readonly UdpMux _udp;
     private readonly TcpAcceptor _tcp;
@@ -34,8 +35,10 @@ public sealed class AgentApp : IDisposable
     private readonly TestRunner _tests;
     private readonly FleetLink _link;
 
-    public AgentApp(AgentConfig config, LoadedNodeIdentity identity, TimeProvider time, INodeLog log)
+    /// <param name="dataDirectory">配置目录（容器里的 <c>/etc/monitor</c>）：宿主机更新器和节点在这里交换清单、请求和状态。</param>
+    public AgentApp(AgentConfig config, LoadedNodeIdentity identity, string dataDirectory, TimeProvider time, INodeLog log)
     {
+        _update = new UpdateBridge(dataDirectory, new ConfigStore(dataDirectory).Write, log);
         _config = config;
         _identity = identity;
         _time = time;
@@ -200,6 +203,24 @@ public sealed class AgentApp : IDisposable
                 (_, cancel, _) => ValueTask.FromResult(Result<TestRunResult>.Ok(new TestRunResult { Accepted = _tests.Cancel(cancel.TestId) })));
         }
 
+        if (Allow(MonitorMethods.UpdateOffer))
+        {
+            router.OnNotify(MonitorMethods.UpdateOffer, MonitorJsonContext.Default.UpdateOfferParams, (_, offer, _) =>
+            {
+                _update.WriteOffer(offer);
+                return ValueTask.CompletedTask;
+            });
+        }
+
+        if (Allow(MonitorMethods.UpdateApply))
+        {
+            router.OnNotify(MonitorMethods.UpdateApply, MonitorJsonContext.Default.UpdateOfferParams, (_, _, _) =>
+            {
+                _update.RequestApply();
+                return ValueTask.CompletedTask;
+            });
+        }
+
         if (Allow(MonitorMethods.ProbeDisconnect))
         {
             router.On(
@@ -270,6 +291,7 @@ public sealed class AgentApp : IDisposable
             DroppedInvalid = _counters.DroppedInvalid,
             DroppedRateLimited = _counters.DroppedRateLimited,
             QueueBytes = _outbox.PendingBytes,
+            Update = _update.ReadStatus(),
         };
         _lastIn = packetsIn;
         _lastOut = packetsOut;

backend/agent/src/Monitor.Agent/Program.cs

@@ -35,7 +35,7 @@ catch (Exception ex) when (ex is IOException or InvalidDataException or InvalidO
 
 using (identity)
 {
-    using var app = new AgentApp(config, identity, time, log);
+    using var app = new AgentApp(config, identity, Path.GetDirectoryName(Path.GetFullPath(configPath))!, time, log);
     await ServiceShell.RunAsync(app.RunAsync, log);
 }