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

gateway/config.py

@@ -550,6 +550,8 @@ def load_gateway_config() -> GatewayConfig:
                     os.environ["DISCORD_FREE_RESPONSE_CHANNELS"] = str(frc)
                 if "auto_thread" in discord_cfg and not os.getenv("DISCORD_AUTO_THREAD"):
                     os.environ["DISCORD_AUTO_THREAD"] = str(discord_cfg["auto_thread"]).lower()
+                if "reactions" in discord_cfg and not os.getenv("DISCORD_REACTIONS"):
+                    os.environ["DISCORD_REACTIONS"] = str(discord_cfg["reactions"]).lower()
 
             # Telegram settings → env vars (env vars take precedence)
             telegram_cfg = yaml_cfg.get("telegram", {})