feat(04-01): create memory module structure and SQLite manager
- Created src/memory module with MemoryManager stub
- Created src/memory/storage subpackage
- Implemented SQLiteManager with connection management and thread safety
- Database schema supports conversations, messages, and metadata
- Includes proper indexing and error handling
Schema:
- conversations table: id, title, timestamps, metadata, session stats
- messages table: id, conversation_id, role, content, importance, embedding_ref
- Foreign key constraints and performance indexes
- Thread-local connections with WAL mode for concurrency
bdba17773c