feat(flowgrpo): add bounded-memory chunked replay backward - #465

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: amannn/action-semantic-pull-request@e32d7e603df1aa1ba07e981f2a23455dee596825. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/