Cleanup: drop vestigial utterance columns, sync story-graph doc

- 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>

migrations/022_drop_vestigial_utterance_columns.sql

@@ -0,0 +1,5 @@
+-- The utterance model settled on parent-only successors (child count decides linear
+-- vs. options), so advances_to_utterance_id was never used; the effect side-effect
+-- hook was reserved but unbuilt. Drop both.
+ALTER TABLE osint.utterances DROP COLUMN IF EXISTS advances_to_utterance_id;
+ALTER TABLE osint.utterances DROP COLUMN IF EXISTS effect;