Complete the stop reason propagation chain so ACP clients can
distinguish end_turn from max_tokens:
- server-chat.ts: emitChatFinal accepts optional stopReason param,
includes it in the final payload, reads it from lifecycle event data
- translator.ts: read stopReason from the final payload instead of
hardcoding end_turn
Chain: LLM API → run.ts (meta.stopReason) → agent.ts (lifecycle event)
→ server-chat.ts (final payload) → ACP translator (PromptResponse)