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/tests/MoShiro.Backend.Tests/ReleaseFeedTests.cs

@@ -58,7 +58,7 @@ public sealed class ReleaseFeedTests : IAsyncLifetime
     }
 
     [Theory]
-    [InlineData("probe", "dev")] // 仓库里没有这个组件的清单
+    [InlineData("unsupported", "dev")] // 仓库里没有这个组件的清单
     [InlineData("agent", "release")] // 没有这个通道
     public async Task NothingThere_MeansNull_NotAnException(string component, string channel)
     {