feat(session): add Close to SessionStore interface

Add Close() error to SessionStore so callers can release resources
through the interface. JSONLBackend already had Close; this adds
a no-op implementation to SessionManager for compatibility.

pkg/session/manager.go

@@ -265,6 +265,12 @@ func (sm *SessionManager) loadSessions() error {
 	return nil
 }
 
+// Close is a no-op for the in-memory SessionManager; it satisfies the
+// SessionStore interface so callers can release resources uniformly.
+func (sm *SessionManager) Close() error {
+	return nil
+}
+
 // SetHistory updates the messages of a session.
 func (sm *SessionManager) SetHistory(key string, history []providers.Message) {
 	sm.mu.Lock()

pkg/session/session_store.go

@@ -15,4 +15,5 @@ type SessionStore interface {
 	SetHistory(key string, history []providers.Message)
 	TruncateHistory(key string, keepLast int)
 	Save(key string) error
+	Close() error
 }