Fix CI: guard starlette imports, asyncio.run(), deprecate datetime.utcnow()

Add centralized ML model configuration

Add SQLite vector backend as default and fix HNSW test skipping

Add SQLiteGraphStore for bounded, persistent graph storage

Add multi-provider memory system with auto-detection

Fix mypy errors and add network timeout handler for flaky CI tests

fix: resolve mypy type errors

fix: improve error handling and add comprehensive test coverage

fix: resolve mypy type errors across codebase

Add hierarchical memory system with graph + vector storage