Adds _reactions_enabled() gating to match Discord (DISCORD_REACTIONS) and
Telegram (TELEGRAM_REACTIONS) pattern. Defaults to true to preserve existing
behavior. Gates at three levels:
- _handle_slack_message: skips _reacting_message_ids registration
- on_processing_start: early return
- on_processing_complete: early return
Also adds config.yaml bridge (slack.reactions) and two new tests.