Prevent auto-create on agent select

Remove automatic new conversation creation when selecting an agent; now new conversations are only created by clicking the "Nova Conversa" button.

src/pages/TestPrompt.tsx

@@ -69,11 +69,6 @@ export default function TestPrompt() {
     }
   }, [messages]);
 
-  // Limpa a conversa ao trocar de agente
-  useEffect(() => {
-    handleNewConversation();
-  }, [selectedAgent]);
-
   const handleNewConversation = () => {
     if (!selectedAgent) return;