feat(bot): BotDriver with mutex, retry cap, and dispatch

Wires Brain to Game: init/onStateChange/dispose lifecycle, in-flight
mutex, 5-attempt retry loop with attemptHistory, resign-on-cap. Also
adds Game.aiOpponent? field to state.ts for Task 5.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/server/src/bot/index.ts

@@ -2,3 +2,6 @@ export type {
   Brain, BrainInput, BrainAction, BrainInitArgs,
   CandidateMove, AttemptHistoryEntry,
 } from './brain.js';
+export { CasualBrain } from './casual-brain.js';
+export { BotDriver } from './driver.js';
+export { legalCandidates } from './candidates.js';