ci: detect default branch coverage by commit

[saagpatel]

What

• Treat default-branch coverage runs as skippable when the checked-out commit matches the compare branch SHA.
• Keep the existing local branch and GitHub ref-name checks as fast paths.

Why

• GitHub Actions can run the default branch in a detached checkout, and the previous env/local-branch checks did not catch the post-merge master run.

Testing

• bash -n scripts/ci/run-diff-coverage.sh
• env -u GITHUB_REF_NAME -u GITHUB_REF bash scripts/ci/run-diff-coverage.sh
• env GITHUB_REF=refs/heads/master bash scripts/ci/run-diff-coverage.sh
• git diff --check scripts/ci/run-diff-coverage.sh