feat: auto-start new topics from bot senders

[DeepColds]

Summary

• Add autoStartOnNewTopicFromBots as an opt-in extension for topic-group new-topic auto-start.
• When enabled, bot-originated top-level messages in topic groups can seed an auto-started session; default remains human-only to avoid unexpected bot loops.
• Expose the toggle in Dashboard Bot Defaults and persist it via existing card-prefs APIs.
• Add unit coverage for pure policy and event dispatcher routing.

Validation

• pnpm vitest run test/auto-start.test.ts test/event-dispatcher.test.ts
• pnpm tsc --noEmit
• git diff --check