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.Link/Transports.cs

@@ -112,7 +112,7 @@ public sealed class WsTransport : IRpcTransport
 }
 
 /// <summary>
-/// TLS 帧传输（Probe → Agent）：<c>u32 LE 正文长度 | u8 类型(0 文本, 1 二进制) | 正文</c>。
+/// TLS 帧传输（拨号方 → 监听方）：<c>u32 LE 正文长度 | u8 类型(0 文本, 1 二进制) | 正文</c>。
 /// 比 WebSocket 轻得多，Agent 不需要内置 HTTP 服务端。
 /// </summary>
 public sealed class TlsFrameTransport : IRpcTransport