Previously, `autoReplyEnabled=false` would block ALL email sends,
including proactive emails triggered from other channels (e.g., asking
nanobot on Feishu to send an email).
Now `autoReplyEnabled` only controls automatic replies to incoming
emails, not proactive sends. This allows users to disable auto-replies
while still being able to ask nanobot to send emails on demand.
Changes:
- Check if recipient is in `_last_subject_by_chat` to determine if
it's a reply
- Only skip sending when it's a reply AND auto_reply_enabled is false
- Add test for proactive send with auto_reply_enabled=false
- Update existing test to verify reply behavior
