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>

server/migrations.integration.test.ts

@@ -33,7 +33,7 @@ suite('PostgreSQL migrations', () => {
     const migrationsDir = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..', 'migrations')
     const firstRun: string[] = []
     await runMigrations(testDatabaseUrl, migrationsDir, message => firstRun.push(message))
-    expect(firstRun.filter(message => message.startsWith('apply '))).toHaveLength(21)
+    expect(firstRun.filter(message => message.startsWith('apply '))).toHaveLength(22)
 
     const client = new Client({ connectionString: testDatabaseUrl })
     await client.connect()
@@ -49,7 +49,7 @@ suite('PostgreSQL migrations', () => {
     ]))
     expect(tableNames).not.toEqual(expect.arrayContaining(['cases', 'widgets', 'widget_relations', 'cutscenes', 'dialogue_steps', 'mystery_chapters', 'seen_dialogue']))
     const ledger = await client.query<{ count: string }>('SELECT COUNT(*)::text AS count FROM osint.schema_migrations')
-    expect(ledger.rows[0].count).toBe('21')
+    expect(ledger.rows[0].count).toBe('22')
     const connectionColumns = await client.query<{ column_name: string }>(`SELECT column_name FROM information_schema.columns WHERE table_schema='osint' AND table_name='exhibit_connections'`)
     expect(connectionColumns.rows.map(row => row.column_name)).toEqual(expect.arrayContaining(['label', 'tightness', 'tag_style', 'tag_position_percent', 'tag_lateral_offset']))
     const eventOccurrence = await client.query<{ is_nullable: string }>(`SELECT is_nullable FROM information_schema.columns WHERE table_schema='osint' AND table_name='event_exhibits' AND column_name='occurred_at'`)
@@ -58,7 +58,7 @@ suite('PostgreSQL migrations', () => {
 
     const secondRun: string[] = []
     await runMigrations(testDatabaseUrl, migrationsDir, message => secondRun.push(message))
-    expect(secondRun.filter(message => message.startsWith('skip  '))).toHaveLength(21)
+    expect(secondRun.filter(message => message.startsWith('skip  '))).toHaveLength(22)
     expect(secondRun.some(message => message.startsWith('apply '))).toBe(false)
   })
 })