agent: add WatchAgent support and notify on /agent change

- Add WatchAgent constant to WaitChange fields
- Add WatchAgent case in WaitChange read() function
- Call notifyChange() in /agent command after setting agentName

This enables the D-Bus adapter to detect agent changes and emit
ConfigChanged signals so the GUI can refresh the session list.

pkg/agent/commands.go

@@ -199,6 +199,7 @@ func (s *agent) handleCommand(ctx context.Context, input string) bool {
 			s.agentName = name
 			s.session = nil
 			s.pushSessionEnv()
+			s.notifyChange()
 			for _, msg := range rt.Messages {
 				s.emit(infoEvent(msg))
 			}

pkg/agent/core.go

@@ -752,6 +752,8 @@ func (s *agent) WaitChange(ctx context.Context, field, current string) (string,
 			return s.CtxSz()
 		case WatchCWD:
 			return s.CWD()
+		case WatchAgent:
+			return s.AgentName()
 		}
 		return ""
 	}

pkg/agent/types.go

@@ -15,6 +15,7 @@ const (
 	WatchUsage = "usage"
 	WatchCtxSz = "ctxsz"
 	WatchCWD   = "cwd"
+	WatchAgent = "agent"
 )
 
 // ErrInterrupted is returned when the user cancels an agent turn (Ctrl-C).