github.rkt reads GITHUB_REF and GITHUB_REPOSITORY at module level, which blocks testing it

[jackfirth]

private/github.rkt is at 61% coverage (48 missed lines). The untested part is review construction: refactoring-result->github-review-comment and refactoring-results->github-review. Two obstacles stand in the way. First, branch-ref and github-repository are module-level getenv calls, so tests can't control them — they should probably become parameters or be read lazily. Second, building a review comment shells out to git ls-tree via git-path, so tests need the temp-repo fixture from #819. The env var situation is a design problem worth fixing independently of coverage.

Part of #680.

[jackfirth]

Reopening since the coverage gap isn't fully addressed until the temp repo setup infrastructure is in place.