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 ran under node 24
and 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. 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

TODO.md

@@ -21,6 +21,21 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-08: The canonical `script/bootstrap` now uses the installed node only
+  when its major version is the pinned one (issue 118). Otherwise, as when node
+  is missing, it installs the pinned node under nvm and installs yarn and the
+  packages under it. The CI runner image ships node 24, under which the pinned
+  yarn 1.22.22 printed the deprecation warning DEP0169 on every bootstrap. Only
+  the major is compared because the `Dockerfile` stages start from a node 22
+  alpine image whose exact version is not the pin, and nvm cannot install a
+  prebuilt node on alpine; the version-comparison rule in `REPO_POLICIES.md`
+  names node as its exception. `script/fmt` and `script/fmt-check` pick their
+  yarn with the same test: the `yarn` on `PATH` when the installed node is the
+  pinned major, and otherwise yarn under the pinned node through nvm.
+  `REPO_POLICIES.md` and both checklists say so, and the checklists' final
+  `script/cibuild` check names what the CI runner image has: node 24 and no
+  yarn. Not yet tried on the shared runner. Repositories pick this up on their
+  next re-vendor.
 - 2026-10-07: The `script/cibuild` item in `NEW_REPO_CHECKLIST.md` now matches
   the canonical `script/cibuild` (issue 125). Its image build carries the tag,
   `-t "$tag"`, and the item says that `$version` comes from