The 'Checkout BEC Plugin Repository' step pinned repository to bec/csaxs_bec
with ref github.head_ref. For a pull request opened from a fork, head_ref is
the fork's branch name, which does not exist in the base repo, so the checkout
failed with 'git ... exit code 1' before any tests ran.
Fall through to github.sha instead: on pull_request events that is the
test-merge commit, which lives in the base repo, so the checkout succeeds for
fork PRs and in-repo branches alike (and gates on the merged result).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>