♻️ Adopt setup: prefix and retire the stray use: script

Introduce a setup: prefix for one-time clone configuration (mutates the
local environment, never a hook or CI step) with setup as its umbrella
aggregator. Move use:git-commit-message to setup:git-commit-message as
its first member, retiring use: — the undocumented prefix tracked in the
backlog. Update both CONTRIBUTING.md prefix lists, the bare-command
inventory, and the stale use: reference in branch.sh.

scripts/branch.sh

@@ -26,7 +26,7 @@ set -eu
 # `create:` was kept because both members really do create something: a branch,
 # a release. It was added to both prefix lists in CONTRIBUTING.md in the same
 # commit as its first members, because a prefix missing from those lists is
-# invisible — which is the `use:` mistake this repo now carries in backlog.tasks.
+# invisible — which was the `use:` mistake this repo carried in backlog.tasks (since retired into `setup:`).
 # There is deliberately no bare `create` aggregator: "run all the workflows"
 # describes nothing anyone wants, and `publish:*` already sets the precedent for
 # a prefix without one.