fix: detect oper status when users join channels

Previously the bot only sent WHO on connect (001), so users joining
after the initial scan were never checked for oper status. Now sends
WHO <nick> on every JOIN event to detect opers mid-session.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/derp/bot.py

@@ -265,6 +265,12 @@ class Bot:
             if "*" in flags:
                 self._opers.add(f"{nick}!{user}@{host}")
 
+        # JOIN — WHO the joining user to detect oper status
+        if msg.command == "JOIN" and msg.nick and msg.nick != self.nick:
+            channel = msg.params[0] if msg.params else ""
+            if channel:
+                await self.conn.send(format_msg("WHO", msg.nick))
+
         # QUIT — remove departed nicks from oper set
         if msg.command == "QUIT" and msg.prefix:
             self._opers.discard(msg.prefix)