Introduce a third Coach memory type alongside facts (durable traits) and
plans (standing if-then habits): a time-anchored "bring this back later"
reminder with a due moment and a done state. The free-text note is
AES-256-GCM ciphertext via the shared Bytes codec; metric, status, the
trigger fields and the dates stay plain so the sweep and the injection
picker rank and filter without a per-row decrypt.
Migration 0196 is purely additive (status and trigger_kind are app-side
closed enums, not DB enums). Register the new note_encrypted column in the
encrypted-columns registry and the rotation script so a key rotation can
never silently skip it.
