fix(agent): failure reason leads with model's diagnosis, not the task prompt

之前聚合用 "<任务描述前80字>: <error>",失败 reason 被整段 prompt 污染,
真正的"看到什么/缺什么"被挤到后面。改为以模型 summary 为主、附上 distinct error,
reason 直接就是诊断本身(例:"No source files found...; only README/jsonl present.
(Required files like src/database/redis/main.js are missing.)")。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/task_executor.py

@@ -126,15 +126,22 @@ class TaskExecutor:
                 "usage": self._usage_payload(time.time() - started_at),
             }
             if not success:
-                # Surface the real per-subtask failure detail as a top-level `error` so the
-                # orchestrator records WHY (agent/main.py falls back to a generic "Task failed"
-                # when this key is absent — see incident 2026-06-15 swarm-69e470561bd7-seed).
+                # Surface the model's OWN failure explanation (what it saw / what was missing) as a
+                # top-level `error`, so the orchestrator/Manager records WHY instead of a generic
+                # "Task failed" (agent/main.py uses this as the failure reason). Lead with the model's
+                # summary/error — NOT the task prompt — so the reason reads as the diagnosis, not the
+                # ask. summary is usually the fuller "saw X, missing Y" narrative; append a distinct
+                # error for the concise cause.
                 failed = [r for r in results if r.get("status") not in ("completed", "handed_off")]
-                detail = "; ".join(
-                    f"{((r.get('subtask') or {}).get('description') or 'subtask')[:80]}: "
-                    f"{r.get('error') or r.get('summary') or 'no detail reported'}"
-                    for r in failed
-                ) or "subtask(s) failed without detail"
+                explanations = []
+                for r in failed:
+                    summary = str(r.get("summary") or "").strip()
+                    error = str(r.get("error") or "").strip()
+                    text = summary or error or "subtask failed without detail"
+                    if summary and error and error not in summary:
+                        text = f"{summary} ({error})"
+                    explanations.append(text)
+                detail = "; ".join(explanations) or "subtask(s) failed without detail"
                 payload["error"] = detail
                 logger.error(f"Task {task_id} failed: {detail}")
             return payload