fix(worker): don't double-append streamed answer when completed event carries content

apps/agent-worker/src/runtime/turn-outcome.ts

@@ -42,3 +42,19 @@ export const resolveTurnOutcome = (args: {
   }
   return { text };
 };
+
+// Pure decision for whether the `content` on an `assistant_completed` event
+// should be folded into the accumulated assistant text.
+//
+// Some runtimes (Claude `-p --output-format stream-json`, OpenCode) emit the
+// final answer TWICE: once as streamed `assistant_delta` chunks and again as
+// the `content` on the terminal `assistant_completed`/`result` event. Appending
+// both yields a doubled "<answer><answer>". So only fold in the completed
+// content when nothing has streamed yet (a runtime that emits only a final
+// result). When deltas already carried the answer, skip it. The truly-empty
+// case is still covered by resolveTurnOutcome's recoveredText/finalMessage
+// fallback, so nothing is lost.
+export const shouldAppendCompletedContent = (
+  currentText: string,
+  completedContent?: string,
+): boolean => Boolean(completedContent) && !currentText.trim();