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/tests/Monitor.Backend.Tests/AgentHarness.cs

@@ -56,7 +56,7 @@ public sealed class AgentHarness : IAsyncDisposable
         };
         configure?.Invoke(config);
         LoadedNodeIdentity identity = LoadedNodeIdentity.Load(directory);
-        var app = new AgentApp(config, identity, TimeProvider.System, new ConsoleNodeLog(TimeProvider.System, Console.Out));
+        var app = new AgentApp(config, identity, directory, TimeProvider.System, new ConsoleNodeLog(TimeProvider.System, Console.Out));
         var harness = new AgentHarness(backend, directory, identity, app);
         await app.Ready.WaitAsync(TimeSpan.FromSeconds(10));
         await BackendFixture.EventuallyAsync(() => Task.FromResult(app.Link.IsConnected), because: "the agent should connect to the backend");