Configures GitHub Actions test workflows - `.github/workflows/test.yml` with matrix builds (OS × runtime, with per-OS quirks - path separators, line endings, shells - and per-language runtime matrices in references/os-matrix.md), JUnit XML artifact upload, retry/sharding, services (PostgreSQL, Redis), per-trigger filtering (pull_request, push, schedule, workflow_dispatch). Use when the project hosts on GitHub and the team wants idiomatic GitHub Actions patterns for test workflows, or needs continuous cross-platform OS / runtime coverage.