fix frontend invoke result handling

frontend/src/a2a.js

@@ -1,5 +1,11 @@
 export const CONFIG_URL = import.meta.env.DEV ? "/app/config.json" : "./config.json";
 
+export function unwrapInvokeResponse(payload) {
+  return payload && typeof payload === "object" && "result" in payload
+    ? payload.result
+    : payload;
+}
+
 export async function requestJson(url, init = {}) {
   const response = await fetch(url, { credentials: "same-origin", ...init });
   const text = await response.text();
@@ -26,7 +32,7 @@ export async function callSkill(config, skillName, args) {
       throw new Error("sign in required");
     }
   }
-  return requestJson(
+  const payload = await requestJson(
     `${config.endpoints.invoke}/${encodeURIComponent(skillName)}`,
     {
       method: "POST",
@@ -34,6 +40,7 @@ export async function callSkill(config, skillName, args) {
       body: JSON.stringify({ arguments: args }),
     },
   );
+  return unwrapInvokeResponse(payload);
 }
 
 export function sampleValue(schema) {