Allows controlling auto-reactions (ack emoji) per-session via sessions.json.
Session-level ackReaction can be set to 'always', 'mentions', or 'never'
to override global direct/group settings.
- Add sessionMode parameter to shouldAckReactionForWhatsApp
- Load session config in maybeSendAckReaction to check override
- Add ackReaction field to SessionEntry type
- Add tests for session override behavior
- Document per-session override in whatsapp.md
