The job summary is the deliverable #83 asked for -- "readable without opening a
log" -- and GitHub exposes no API that reads a job summary back: the check-run
output for the advisory job returns summary: null, so a write that silently did
not happen would be invisible to everything except a human on the run page. The
step log can be read, so it now carries one line saying which of the two
happened, including the case where GITHUB_STEP_SUMMARY is unset entirely, which
is what running the script by hand looks like.