The probe used --heads with a bare name, so it could not describe a tag push
and would have fallen back to develop for one. Resolve a fully qualified ref
from the event instead: the PR base or pushed branch under refs/heads, a tag
under refs/tags, and fail loudly on an unrecognised ref type.
Only branch refs are eligible for the develop fallback. A tag that is absent
from frappe is a real error, not a stacked-PR base, so it still fails.