fix: address Phase 5 review — use strict equality check in cmdMode

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/bot/instance.ts

@@ -338,7 +338,7 @@ export class BotInstance extends EventEmitter {
       rloop: PlayMode.RandomLoop,
     };
     const mode = modeMap[cmd.args];
-    if (!mode) return "Usage: !mode <seq|loop|random|rloop>";
+    if (mode === undefined) return "Usage: !mode <seq|loop|random|rloop>";
     this.queue.setMode(mode);
     this.emit("stateChange");
     return `Play mode set to: ${cmd.args}`;