feat: add channel-based filtering for Discord

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>

nanobot/channels/discord.py

@@ -53,6 +53,7 @@ class DiscordConfig(Base):
     enabled: bool = False
     token: str = ""
     allow_from: list[str] = Field(default_factory=list)
+    allow_channels: list[str] = Field(default_factory=list)  # Allowed channel IDs (empty = all)
     intents: int = 37377
     group_policy: Literal["mention", "open"] = "mention"
     read_receipt_emoji: str = "👀"
@@ -533,6 +534,12 @@ class DiscordChannel(BaseChannel):
         """Check if inbound Discord message should be processed."""
         if not self.is_allowed(sender_id):
             return False
+        # Channel-based filtering: only respond in allowed channels
+        allow_channels = self.config.allow_channels
+        if allow_channels:
+            channel_id = self._channel_key(message.channel)
+            if channel_id not in allow_channels:
+                return False
         if message.guild is not None and not self._should_respond_in_group(message, content):
             return False
         return True