fix: session persistence across restart

Shutdown() was calling KillSession() which deleted the persisted
JSON files immediately after saveAllSessions() wrote them. Now
Shutdown() closes sessions directly without removing persistence
files, allowing restoreAllSessions() to find them on next start.

session/session.go

@@ -803,7 +803,15 @@ func (s *Manager) Shutdown() {
 	}
 	s.mu.Unlock()
 	for _, id := range ids {
-		s.KillSession(id)
+		s.mu.Lock()
+		sess := s.sessions[id]
+		delete(s.sessions, id)
+		s.mu.Unlock()
+		if sess != nil {
+			sess.Cancel()
+			sess.Core.Close()
+			s.cfg.Log.Info("shutdown session %s", id)
+		}
 	}
 }