feat: exit listener after successful switch for automatic restart

After a successful switch deployment, the listener now exits gracefully
so systemd can restart it with the new binary. This works together with
stopIfChanged/restartIfChanged to ensure deployments complete before
the service restarts.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/listener/listener.go

@@ -34,6 +34,10 @@ type Listener struct {
 
 	// Expanded subjects for discovery responses
 	expandedSubjects []string
+
+	// restartCh signals that the listener should exit for restart
+	// (e.g., after a successful switch deployment)
+	restartCh chan struct{}
 }
 
 // New creates a new listener with the given configuration.
@@ -43,10 +47,11 @@ func New(cfg Config, logger *slog.Logger) *Listener {
 	}
 
 	return &Listener{
-		cfg:      cfg,
-		executor: deploy.NewExecutor(cfg.FlakeURL, cfg.Hostname, cfg.Timeout),
-		lock:     deploy.NewLock(),
-		logger:   logger,
+		cfg:       cfg,
+		executor:  deploy.NewExecutor(cfg.FlakeURL, cfg.Hostname, cfg.Timeout),
+		lock:      deploy.NewLock(),
+		logger:    logger,
+		restartCh: make(chan struct{}, 1),
 	}
 }
 
@@ -93,9 +98,13 @@ func (l *Listener) Run(ctx context.Context) error {
 
 	l.logger.Info("listener started", "deploy_subjects", l.expandedSubjects, "discover_subject", discoverSubject)
 
-	// Wait for context cancellation
-	<-ctx.Done()
-	l.logger.Info("shutting down listener")
+	// Wait for context cancellation or restart signal
+	select {
+	case <-ctx.Done():
+		l.logger.Info("shutting down listener")
+	case <-l.restartCh:
+		l.logger.Info("exiting for restart after successful switch deployment")
+	}
 
 	return nil
 }
@@ -185,6 +194,15 @@ func (l *Listener) handleDeployRequest(subject string, data []byte) {
 			messages.StatusCompleted,
 			"deployment completed successfully",
 		))
+
+		// After a successful switch, signal restart so we pick up any new version
+		if req.Action == messages.ActionSwitch {
+			select {
+			case l.restartCh <- struct{}{}:
+			default:
+				// Channel already has a signal pending
+			}
+		}
 	} else {
 		l.logger.Error("deployment failed",
 			"exit_code", result.ExitCode,