♻️ 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.

CONTRIBUTING.md

@@ -7,7 +7,7 @@ This document is for maintainers and contributors working on the project itself.
 CI and review will bounce these even though `npm run check` and the linters don't catch them. They're the high-frequency things a contributor (or an agent) reaches for by default:
 
 - **Source imports use `.ts` extensions, never `.js`.** `node --strip-types` resolves the `.ts` form at test time; `rewriteRelativeImportExtensions` emits `.js` in `dist/`. "Pre-fixing" an import to `.js` breaks the inner loop. (rationale: README § Tooling decisions)
-- **A new `npm run` script must reuse an existing prefix** (`create:` / `check:` / `fix:` / `test:` / `watch:` / `maintain:` / `publish:`). If none fits, that's a signal the script doesn't belong in the pipeline — not a reason to invent a new prefix. If it genuinely does belong, add the prefix to both lists here in the same commit; an undocumented prefix becomes invisible and quietly accrues members. (see [Script prefix convention](#script-prefix-convention))
+- **A new `npm run` script must reuse an existing prefix** (`create:` / `check:` / `fix:` / `test:` / `watch:` / `maintain:` / `publish:` / `setup:`). If none fits, that's a signal the script doesn't belong in the pipeline — not a reason to invent a new prefix. If it genuinely does belong, add the prefix to both lists here in the same commit; an undocumented prefix becomes invisible and quietly accrues members. (see [Script prefix convention](#script-prefix-convention))
 - **`oxlint-disable` directives live in source, not `.oxlintrc.json`.** The trade-off must sit next to the code it silences. This is a _human_ last-resort convention; agents must not add these — see [AGENTS.md § Never do](./AGENTS.md#never-do). (rationale: README § Tooling decisions)
 - **Don't put slow / network / whole-project scans in `check` or pre-commit.** Advisory scans are not correctness gates; they belong under `maintain:`. (see [Feedback tiers](#feedback-tiers) and [Script prefix convention](#script-prefix-convention))
 - **New work starts with `npm run create:branch`, never a hand-written `git switch -c`/`git checkout -b`.** The command carries the branch precondition; branching around it skips the clean-tree, current-`main` and green-baseline checks, and the skip is invisible until a failure can no longer be attributed. (see [Branching model](#branching-model))
@@ -15,7 +15,7 @@ CI and review will bounce these even though `npm run check` and the linters don'
 
 ## Commit messages
 
-Gitmoji subject, imperative mood, 50/72 wrapping. The template is `commit-message-template`; run `npm run use:git-commit-message` once after cloning to register it as git's `commit.template`.
+Gitmoji subject, imperative mood, 50/72 wrapping. The template is `commit-message-template`; run `npm run setup:git-commit-message` once after cloning to register it as git's `commit.template` (or `npm run setup` to run every one-time clone step).
 
 Examples from history: `:sparkles: Add watch tier with watch:test child`, `:recycle: Move type-aware config to .oxlintrc.json; use source-level disable directives`, `:memo: Restore unique maintainer content as CONTRIBUTING.md`. The body explains _what and why_, not _how_; link issues with `Resolves #...`.
 
@@ -30,10 +30,11 @@ Script names in `package.json` use a prefix that signals _when_ the script is in
 - `watch:*` — long-running watchers for the manual inner dev loop. Aggregated by `watch`; currently a single child (`watch:test`), and a future `watch:oxlint` / `watch:tsc` would run concurrently under that umbrella.
 - `maintain:*` — advisory repo-maintenance scans: read-only, but whole-project and/or network-bound, so never a correctness gate. Aggregated by `npm run maintain`.
 - `publish:*` — validates the _publishable artifact_ (e.g. `dist/`) rather than the source, so it needs a fresh build. (see [Rules the tools don't enforce](#rules-the-tools-dont-enforce) and [Publishing workflow](#publishing-workflow))
+- `setup:*` — one-time configuration of a fresh clone; mutates the local environment (git config, editor settings) rather than the repo source, so it is never part of a hook or CI step. Aggregated by `npm run setup` (the umbrella), run once after cloning.
 
-A new script should pick the prefix that matches its lifecycle, not invent a new one. If no existing prefix fits, that's a signal the script doesn't belong in the standard pipeline. When it genuinely does belong, a new prefix is allowed — but it enters both lists in this file in the same commit as its first member, otherwise the rule "reuse an existing prefix" silently develops an exception (`use:` is tracked as exactly that in `backlog.tasks`).
+A new script should pick the prefix that matches its lifecycle, not invent a new one. If no existing prefix fits, that's a signal the script doesn't belong in the standard pipeline. When it genuinely does belong, a new prefix is allowed — but it enters both lists in this file in the same commit as its first member, otherwise the rule "reuse an existing prefix" silently develops an exception (the old `use:` prefix was exactly that; it is now retired into `setup:`).
 
-Separately, some top-level scripts are **bare** (no prefix): the entry points that either run a single tool (`build`, `clean`) or aggregate a `prefix:*` family (`check`, `fix`, `test`, `watch`, `maintain`), plus one convenience that composes across tiers: `verify` — composing `check` + `test:unit` into one whole-project correctness gate (it deliberately uses `test:unit` rather than `test` because `check` already runs `check:tsc`, so the type checker runs exactly once). Bare commands are how you invoke a tier; the `prefix:*` scripts are what those tiers are made of.
+Separately, some top-level scripts are **bare** (no prefix): the entry points that either run a single tool (`build`, `clean`) or aggregate a `prefix:*` family (`check`, `fix`, `test`, `watch`, `maintain`, `setup`), plus one convenience that composes across tiers: `verify` — composing `check` + `test:unit` into one whole-project correctness gate (it deliberately uses `test:unit` rather than `test` because `check` already runs `check:tsc`, so the type checker runs exactly once). Bare commands are how you invoke a tier; the `prefix:*` scripts are what those tiers are made of.
 
 ## Feedback tiers