Add `facts` table to schema with columns for id, agent_id, content,
category, confidence, source_session, and created_at. Implement
store_facts() and get_top_facts() on MemoryStore using upsert-by-id
and confidence-desc ordering. Facts extracted from conversations are
now durable across sessions.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>