Implements KnowledgeStore (extends MemoryBackend) for Deep Research with a
rich SQLite/FTS5 schema supporting source, doc_type, author, participants,
timestamp, thread_id, url, and chunk_index columns; BM25 ranking via FTS5
with porter unicode61 tokenizer; filtered retrieval by source, doc_type,
author, since, and until; MEMORY_STORE/MEMORY_RETRIEVE event emission; WAL
journal mode; and 15 isolated tests using tmp_path fixtures.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>