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/TrustTests.cs

@@ -33,7 +33,7 @@ public sealed class TrustTests : IClassFixture<BackendFixture>
     private static JsonObject Sign(SoftwarePasskey passkey, string statement) => SharedOperator.Sign(passkey, statement);
 
     /// <summary>创建一个加入码（普通 API key 就行，不需要 passkey），返回响应里的码和三条安装命令。</summary>
-    private async Task<(string Code, JsonNode Commands, HttpStatusCode Status)> CreateCodeAsync(string role = "probe", string[]? controllers = null)
+    private async Task<(string Code, JsonNode Commands, HttpStatusCode Status)> CreateCodeAsync(string role = "agent", string[]? controllers = null)
     {
         var body = new JsonObject { ["role"] = role, ["note"] = "trust test" };
         if (controllers is not null)