feat: add per-channel plugin filtering

Channels with a [channels."#name"] section and `plugins` list only run
those plugins. Unconfigured channels run everything. Core is always
active. PMs are unrestricted. Denied commands are silently ignored.

src/derp/bot.py

@@ -281,8 +281,11 @@ class Bot:
             return
 
         # Dispatch to event handlers (fire-and-forget)
+        channel = msg.target if msg.is_channel else None
         event_type = msg.command
         for handler in self.registry.events.get(event_type, []):
+            if not self._plugin_allowed(handler.plugin, channel):
+                continue
             self._spawn(handler.callback(self, msg), name=f"event:{handler.name}")
 
         # Dispatch to command handlers (PRIVMSG only)
@@ -312,6 +315,20 @@ class Bot:
         await self.conn.send(format_msg("NOTICE", msg.nick, reply))
         log.debug("CTCP %s reply to %s", ctcp_cmd, msg.nick)
 
+    def _plugin_allowed(self, plugin_name: str, channel: str | None) -> bool:
+        """Check if a plugin is allowed in a channel."""
+        if plugin_name == "core":
+            return True
+        if not channel or not channel.startswith(("#", "&")):
+            return True  # PMs are unrestricted
+        chan_cfg = self.config.get("channels", {}).get(channel)
+        if not chan_cfg:
+            return True  # unconfigured channels run everything
+        allowed = chan_cfg.get("plugins")
+        if allowed is None:
+            return True
+        return plugin_name in allowed
+
     def _is_admin(self, msg: Message) -> bool:
         """Check if the message sender is a bot admin.
 
@@ -345,6 +362,10 @@ class Bot:
                          name=f"cmd:{cmd_name}:ambiguous")
             return
 
+        channel = msg.target if msg.is_channel else None
+        if not self._plugin_allowed(handler.plugin, channel):
+            return
+
         if handler.admin and not self._is_admin(msg):
             deny = f"Permission denied: {self.prefix}{cmd_name} requires admin"
             self._spawn(self.reply(msg, deny), name=f"cmd:{cmd_name}:denied")

src/derp/config.py

@@ -29,8 +29,10 @@ DEFAULTS: dict = {
         "rate_burst": 5,
         "admins": [],
     },
+    "channels": {},
     "logging": {
         "level": "info",
+        "format": "text",
     },
 }