Verify a pre-commit hook that is supposed to REFUSE commits, without
destroying the hook while testing it. Use when: (1) you just wrote a
pre-commit hook and need to prove it blocks and allows the right things,
(2) a hook you wrote reports "nothing changed" during a commit even though
files are obviously staged, (3) you ran `git reset --hard HEAD~1` to undo a
test commit and lost real work that was in the same commit, (4) you need to
recover a commit you just hard-reset away. Covers the staged-vs-committed
diff trap, safe test sequencing, and reflog recovery.
