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>

src/utteranceCanvas.tsx

@@ -3,8 +3,8 @@ import { useCallback, useEffect, useRef, useState, type ReactElement } from 'rea
 type Utterer = 'npc' | 'player'
 type Utterance = {
   id: string; nodeId: string; utterer: Utterer; npcId: string | null; poseKey: string | null; text: string
-  parentUtteranceId: string | null; advancesToUtteranceId: string | null; terminalId: string | null
-  effect: string | null; xpos: number; ypos: number; sortOrder: number
+  parentUtteranceId: string | null; terminalId: string | null
+  xpos: number; ypos: number; sortOrder: number
 }
 type Terminal = { id: string; terminalKey: string; label: string }
 type Npc = { id: string; name: string; poses: { poseKey: string; url: string }[] }