♻️ 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.

backlog.tasks

@@ -18,9 +18,9 @@ Setup:
     - maybe skills?
     → resolved: adopted @spences10/pi-lsp@0.0.46 as a project-local pi extension (`.pi/settings.json`). It auto-detects the repo's TypeScript 7 (no `lib/tsserver.js`) and spawns `tsc --lsp --stdio` — the same tsgo binary VS Code's native-preview uses. Read-only: hover / definition / references / symbols / diagnostics. No skill; skills don't own persistent processes.
 
-☐ Adopt a `setup:` prefix for one-time clone configuration
-    ☐ Register `use:git-commit-message` as its first member (and retire `use:` — it is in no prefix list)
-    ☐ Create an umbrella 'setup' task that runs all setup tasks - currently one
+✔ Adopt a `setup:` prefix for one-time clone configuration @done
+    ✔ Register `use:git-commit-message` as its first member (and retire `use:` — it is in no prefix list) @done
+    ✔ Create an umbrella 'setup' task that runs all setup tasks - currently one @done
 
 v1.0:
 ☐ API surface is stable and fully typed