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/fleet/src/Fleet.Core/NodeSupport.cs

@@ -9,7 +9,7 @@ public enum NodeLogLevel
     Error,
 }
 
-/// <summary>节点日志：字段名统一为 logfmt（<c>ts level msg agent probe session task result err</c>）。逐包事件不记录，只累加计数。</summary>
+/// <summary>节点日志：字段名统一为 logfmt（<c>ts level msg agent dialer session task result err</c>）。逐包事件不记录，只累加计数。</summary>
 public interface INodeLog
 {
     void Write(NodeLogLevel level, string message, params (string Key, object? Value)[] fields);