fix(tier2): C1 trace 加 workdir 存在守卫(免 scaffold 前刷 FileNotFoundError)

008 实证:首轮 scaffold_init 之前会有早期 SSE 事件,此时工程 workdir 还没建 → trace 追加写刷
FileNotFoundError(best-effort 已吞、但刷屏)。加守卫:dirname 不存在即静默跳过(不抢先建目录、
免干扰 scaffold 的"已存在不覆盖"逻辑)。scaffold 后正常落 trace。

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

tier2/gen-worker/service/control_plane.py

@@ -258,6 +258,11 @@ def _trace_sse_event(trace_path: Any, event: dict, elapsed_s: float, attempt: in
         return
     try:
         import json as _json  # noqa: PLC0415
+        import os as _os  # noqa: PLC0415
+        # 工程 workdir 尚未建(scaffold_init 之前的早期事件)→ 静默跳过,不抢先建目录(免干扰 scaffold 的
+        #   "已存在不覆盖" 逻辑),也免刷 FileNotFoundError(008 实证:首轮 scaffold 前会有这类早期事件)。
+        if not _os.path.isdir(_os.path.dirname(trace_path)):
+            return
         rec = {"t": round(elapsed_s, 1), "attempt": attempt, **_compact_sse_event(event)}
         with open(trace_path, "a", encoding="utf-8") as f:
             f.write(_json.dumps(rec, ensure_ascii=False) + "\n")