detect-backend-changes diffed the event payload's base.sha against the
checked-out ref. Those are two different points in time: actions/checkout
resolves refs/pull/N/merge, and GitHub recomputes that ref whenever the base
branch advances, so the diff picked up whatever landed on staging between the
event firing and the job starting. On a recent UI-only pull request three
backend commits from staging were attributed to the branch, and every backend
shard ran in full
Ask the API which files the pull request touches instead. That is the same set
the Files changed tab shows, and it is immune to either endpoint moving. The
shell body moves into .github/scripts/detect_backend_changes.sh so it can be
exercised directly, and the fail-open paths now also cover an API failure, a
file list past the API's 3000-entry listing ceiling, and a classifier that
prints something unexpected
