Retrieve a fact by its exact key — deterministic, never misses. Use get_fact for self-notes: get_fact(key='agent::self-note:opening') or get_fact(key='agent::self-note:closing'). search_facts misses self-notes because their embedding similarity scores below other identity facts — get_fact(key=) is the only reliable retrieval path. Also use for inter-agent handoffs when the exact key is known. Returns the current (non-deprecated) version of the fact.