agent: unify listening and dialing with directed mutual measurements

Replace Probe with Agent across node runtime, Fleet contracts, backend state, API and storage. Report incoming sessions and outgoing link health through each Agent uplink, and retain directed link controls and retry.

Update protocol fixtures and end-to-end coverage for the unified model; accept requested cancellation during the Fleet timeout test cleanup.

backend/src/MoShiro.Backend/Program.cs

@@ -1,7 +1,7 @@
 using MoShiro.Backend;
 
 WebApplication app = BackendHost.Build(args);
-// 启动时打一行构建信息：版本、tag、提交、构建时间（和 Agent、Probe 启动时打的是同一组）
+// 启动时打一行构建信息：版本、tag、提交、构建时间（和 Agent 启动时打的是同一组）
 if (app.Logger.IsEnabled(LogLevel.Information))
 {
     app.Logger.LogInformation("moshiro backend starting: version {BuildInfo}", MoShiro.Protocol.BuildInfo.Summary);