Adds a 'reactions' key under the discord config section (default: true).
When set to false, the bot no longer adds 👀/✅/❌ reactions to messages
during processing. The config maps to DISCORD_REACTIONS env var following
the same pattern as require_mention and auto_thread.
Files changed:
- hermes_cli/config.py: Add reactions default to DEFAULT_CONFIG
- gateway/config.py: Map discord.reactions to DISCORD_REACTIONS env var
- gateway/platforms/discord.py: Gate on_processing_start/complete hooks
- tests/gateway/test_discord_reactions.py: 3 new tests for config gate