feat: add discord.reactions config option to disable message reactions (#4199)

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

hermes_cli/config.py

@@ -452,6 +452,7 @@ DEFAULT_CONFIG = {
         "require_mention": True,       # Require @mention to respond in server channels
         "free_response_channels": "",  # Comma-separated channel IDs where bot responds without mention
         "auto_thread": True,           # Auto-create threads on @mention in channels (like Slack)
+        "reactions": True,             # Add 👀/✅/❌ reactions to messages during processing
     },
 
     # WhatsApp platform settings (gateway mode)