Previously the Discord channel dropped every message from any bot
account via `if message.author.bot`, which prevented legitimate
multi-agent setups (one bot asking another for help, bot-to-bot
@mentions, etc.) from working.
Narrow the guard to only drop messages from this bot's own account
by comparing against self._bot_user_id (already populated in on_ready).
Self-loop protection is preserved — each bot instance still ignores
its own outbound messages.
Co-authored with Claude Opus 4.7
Alfredo ArenasandXubin Ren
3fd24c72fd