feat: print build info at startup; version placeholders filled in by CI [CI SKIP]

backend/agent/src/MoShiro.Agent/AgentApp.cs

@@ -143,6 +143,7 @@ public sealed class AgentApp : IDisposable
 
     public async Task RunAsync(CancellationToken ct)
     {
+        _log.Info("moshiro agent starting", MoShiro.Protocol.BuildInfo.Fields);
         using CancellationTokenSource linked = CancellationTokenSource.CreateLinkedTokenSource(ct);
         // 数据面和控制端口先开始监听，再连接 Backend：hello 里上报的是实际端口
         _udp.Bind();