fix: read starttedAt with mutex on lifecycle

internal/session/lifecycle/lifecycle.go

@@ -181,5 +181,7 @@ func isClosedError(err error) bool {
 }
 
 func (l *lifecycle) StartedAt() time.Time {
+	l.mu.Lock()
+	defer l.mu.Unlock()
 	return l.startedAt
 }