fix(secrets): reject a --diff ref that starts with "-" - #267

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/

Internal PR into main: extended matrix NOT run — e2e-node, secret-detection-accuracy, sarif-validation, backend-api, package-integrity, cross-platform. Unit tests (test-node, test-python) still run. To run everything, push to a branch and open the PR from a fork, or use workflow_dispatch.