Move the SC2016 / -S warning rationale to the PR body — a decision
justification, not something that needs to live inline forever. Keep
only the one line that explains the non-obvious part of the code
itself: shebang-based discovery over a *.sh glob.
Also make the no-files case explicit: an empty match previously fell
through to shellcheck with no arguments (a confusing usage error, but
still non-zero); now it prints a clear message and exits 1.
f4cabd117b