Add user_id parameter to all ThreadMetaStore abstract methods. Implement
owner isolation in MemoryThreadMetaStore with _get_owned_record helper.
Add check_access to base class and memory implementation. Add
make_thread_store factory to simplify deps.py initialization. Add
memory-backend isolation tests.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>