- migration 022 drops utterances.advances_to_utterance_id and .effect, which the
parent-only/child-count dialogue model never used; purge their references.
- rewrite docs/story-graph.md to match what was built (parent-child utterances,
vertical mystery graph, graph runtime, gate stubs).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
