goal evaluator 的 Stop 钩子在评估失败时会以 fail-open 方式触发

缺陷：目标评估器的 Stop 钩子在评估失败时会 fail-open。现象：/goal clear 删除目标后，每次会话停止仍会触发 "Goal evaluator failed: response was not valid JSON. Treat the goal as incomplete and continue working toward it。"，无限阻止停止。根本原因（引自 app.asar.unpacked/src-tauri/binaries/Claude-sidecar-aarch64-apple-darwin）：