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/tools/fleet-enroll/Program.cs

@@ -2,7 +2,7 @@ using System.Text.Json.Nodes;
 using Fleet.Core;
 using Fleet.Enrollment.Client;
 
-// fleet-enroll --server https://panel --join jt_xxx --name jp-1 --identity-dir /etc/moshiro [--meta '{"probe_tcp_port":7443,...}'] [--version 1.0]
+// fleet-enroll --server https://panel --join jt_xxx --name jp-1 --identity-dir /etc/moshiro [--meta '{"tcp_port":7443,...}'] [--version 1.0]
 // 一次性运行：生成身份、注册、写入身份目录，然后退出。
 string? Arg(string name) => args.SkipWhile(a => a != name).Skip(1).FirstOrDefault();