feat(agent): item 12 子代理自动后台化阈值 FUSION_SUBAGENT_AUTO_BACKGROUND_MS (CC 2.1.198/232) - #103

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/