Title: Bot replies always use UTC timestamps

Description: The bot includes a timestamp in the message envelope sent to the LLM, but it uses .toISOString() which forces UTC. This causes the LLM to think it's hours arguably incorrectly (e.g. 3:00 AM instead of 8:30 AM local time), even if the server timezone is set correctly.

Location:
src/auto-reply/envelope.ts

Fix: Use local time formatting instead of ISO string.