Fix agent output template

Author: ajhai

llmstack/client/src/pages/AppEdit.jsx

@@ -263,7 +263,9 @@ export default function AppEditPage(props) {
               ]
             : appInputFields,
         output_template:
-          app?.type?.slug === "agent" ? "{{agent}}" : appOutputTemplate,
+          app?.type?.slug === "agent"
+            ? { markdown: "{{agent}}" }
+            : appOutputTemplate,
         web_config: app?.web_config || {},
         slack_config: app?.slack_config || {},
         discord_config: app?.discord_config || {},