The CI workflow_dispatch event provides no head_commit, so
${{ github.event.head_commit.message }} expands to empty.
ci-version-bump.sh requires a commit message and exits 1 if absent,
causing manually-triggered CI runs to fail in the version-bump step
even though the actual tests passed.
Fix: fall back to `git log -1 --pretty=%B` when the GitHub event
context provides no commit message. Maintains push-event behavior
unchanged (head_commit.message still primary source).
Observed in CI #615 (manual trigger after PR #28 merge): tests passed,
build OK, but version-bump failed → entire workflow marked FAILURE.
🤖 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
