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/src/MoShiro.Backend.Application/Agents/HostStatusService.cs

@@ -4,7 +4,7 @@ using MoShiro.Protocol;
 namespace MoShiro.Backend.Application;
 
 /// <summary>
-/// <c>moshiro.host</c>：Agent 和 Probe 每隔几秒上报的主机资源占用快照。最新值放在内存里并实时推给订阅者；每个节点每分钟存一个样本做历史。
+/// <c>moshiro.host</c>：Agent 每隔几秒上报的主机资源占用快照。最新值放在内存里并实时推给订阅者；每个节点每分钟存一个样本做历史。
 /// 快照里没有 CPU 型号、主机名、网卡名和 IP（节点根本不采集它们）。
 /// </summary>
 public sealed class HostStatusService