scripts/git-hooks/pre-push refuses a delete, a remote tip missing locally and
any update whose remote tip isn't an ancestor of the local tip; a new branch
and a fast-forward pass. scripts/install-git-hooks.sh puts it in the common
.git/hooks, refuses a set core.hooksPath or a different hook without
--force-install, and runs a canary through git hook run. scripts/slot-busy.sh
exits 0 CLEAR, 1 BUSY, 2 error. scripts/test-git-guard.sh (61 cases) and the
mutant runner in agents/darkwing/work/queue-211; TOOLS.md gets the usage.
Not run yet: written during Gander's slot (sage-2801).
(cherry picked from commit 20e885dc35)