Push Context Events

Push typed, visibility-scoped context for a session (out-of-band from messages). The engine ingests it into its EXISTING speakable/private fact slots (and proposes ``memory_candidates`` to the world model) — NOT raw prompt pass-through. Context is active for its TTL window (default ~10 min) and shapes later replies in this session. ``speakable_facts`` may be said; ``private_facts``/``forbidden_to_reveal`` shape the reply but are kept out of it by the leak guard. Idempotent per ``idempotency_key``.