Replace the simple DISCORD_IGNORE_NO_MENTION check with bot-aware
multi-agent filtering. When multiple agents share a channel:
- If other bots are @mentioned but this bot is not → stay silent
- If only humans are mentioned but not this bot → stay silent
- Messages with no mentions still flow to _handle_message for the
existing DISCORD_REQUIRE_MENTION check
- DMs are unaffected (always handled)
This prevents both agents from responding when only one is addressed.