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/shared/MoShiro.Protocol/Tasks/TaskSet.cs

@@ -34,10 +34,10 @@ public sealed class TaskSpec
 
 public sealed class PeerSpec
 {
-    [JsonPropertyName("probe_id")]
-    public ProbeId ProbeId { get; set; }
+    [JsonPropertyName("node_id")]
+    public NodeId NodeId { get; set; }
 
-    /// <summary>Probe 的身份公钥（SPKI，base64）。</summary>
+    /// <summary>拨号方的身份公钥（SPKI，base64）。</summary>
     [JsonPropertyName("pubkey")]
     public string PublicKey { get; set; } = "";
 }
@@ -53,7 +53,7 @@ public sealed class TaskSet
     [JsonPropertyName("tasks")]
     public List<TaskSpec> Tasks { get; set; } = [];
 
-    /// <summary>允许连接到这个 Agent 的 Probe 公钥。</summary>
+    /// <summary>允许拨入这个节点的拨号方公钥。</summary>
     [JsonPropertyName("peers")]
     public List<PeerSpec> Peers { get; set; } = [];