Add GitHub Actions CI that mirrors the BCR presubmit matrix (default,
prof-libunwind, prof-libgcc, prof-gcc, examples) across 4 platforms and
Bazel 7/8/9/latest. Add publish-to-bcr workflow triggered by GitHub
releases to automate BCR PR submission.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
