Install the pinned node when the installed one is another major version (closes #118)

script/bootstrap used whatever node was installed, so on the CI runner
image, which ships node 24, the pinned yarn 1.22.22 printed the
deprecation warning DEP0169 on every bootstrap. It now uses the
installed node only when its major version is the pinned one, and
otherwise installs the pinned node with nvm, as it already did when node
was missing, with yarn and the packages under it. It prints the node
version it uses. Only the major is compared because the Dockerfile
stages use a node 22 alpine image that nvm cannot replace. script/fmt
and script/fmt-check carry the same test to pick their yarn: the yarn on
PATH, or yarn under the pinned node loaded through nvm.

Model: opus-5-5

prompts/EXISTING_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: Existing Repo Checklist
-last_modified: 2026-10-07
+last_modified: 2026-10-08
 ---
 
 Use this checklist when beginning work in a repo that may not yet conform to our
@@ -149,11 +149,13 @@ with your task.
       the image with `--no-cache`. Without the bootstrap the CI run dies in
       `script/fmt-check`, which runs the formatter on the host and finds nothing
       installed.
-- [ ] `script/fmt` and `script/fmt-check` source nvm for the pinned node version
-      before invoking `yarn`, as `script/bootstrap`'s own install step does.
-      `script/bootstrap` leaves the node and yarn it installs off the `PATH` of
-      the shell that called it, so a bare `yarn` exits 127 on a runner carrying
-      nothing but docker and git.
+- [ ] `script/fmt` and `script/fmt-check` pick their `yarn` with the same test
+      as `script/bootstrap`: the `yarn` on `PATH` when the node on `PATH` has
+      the pinned major version, and otherwise yarn under the pinned node, with
+      nvm loaded through `$HOME/.nvm/nvm.sh`. `script/bootstrap` leaves the node
+      and yarn it installs under nvm off the `PATH` of the shell that called it,
+      so a bare `yarn` exits 127 on a runner carrying nothing but docker and
+      git, or runs under another node.
 - [ ] `script/bootstrap` installs no linter of its own — delete the block, its
       version variables and its call site. A JS repo's `yarn install` stays; it
       brings a linter along with every other dependency, and no verdict is taken
@@ -206,10 +208,11 @@ with your task.
 # Final
 
 - [ ] `make check` passes
-- [ ] `script/cibuild` succeeds in a fresh clone on a host carrying nothing but
-      docker and git, with no node or yarn on `PATH`, which is what CI has, and
-      demonstrably executed the checks — a sub-second build, or `CACHED` on a
-      gate layer, means nothing ran
+- [ ] `script/cibuild` succeeds in a fresh clone on a host carrying what CI has
+      (the runner image `docker.gitea.com/runner-images:ubuntu-latest`: docker,
+      git and node 24, with no yarn on `PATH`), and demonstrably executed the
+      checks — a sub-second build, or `CACHED` on a gate layer, means nothing
+      ran
 - [ ] A planted lint violation fails both `make lint` and a plain
       `docker build .`; revert it afterwards
 - [ ] Commit and merge fixes before starting your actual task