Add `allow_channels` config option to DiscordConfig that restricts
bot responses to specific Discord channels. When the list is empty
(default), the bot responds in all channels (backward compatible).
- Add `allow_channels: list[str]` field to DiscordConfig schema
- Add channel ID check in _handle_message_create after user filtering
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
