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.Server.Abstractions/FleetServerOptions.cs

@@ -5,7 +5,7 @@ public sealed class FleetServerOptions
     /// <summary>节点 ID 的类型前缀，MoShiro 使用 <c>agt</c>。</summary>
     public string NodeIdPrefix { get; set; } = "node";
 
-    /// <summary>加入令牌的「种类」对应的节点 ID 前缀（例如 <c>probe</c> → <c>prb</c>）；没有列出的种类用 <see cref="NodeIdPrefix"/>。</summary>
+    /// <summary>加入令牌的「种类」对应的节点 ID 前缀（例如 <c>edge</c> → <c>edg</c>）；没有列出的种类用 <see cref="NodeIdPrefix"/>。</summary>
     public Dictionary<string, string> NodeIdPrefixes { get; } = new(StringComparer.Ordinal);
 
     public int MaxFrameBytes { get; set; } = 16 << 20;