Bootstrap keeps yarn and node only at their pinned versions (closes #154)

script/bootstrap kept any yarn found on PATH, so YARN_VERSION applied
only where none was installed. It now keeps the yarn on PATH only when
yarn --version is exactly YARN_VERSION, and otherwise prepares that
version through corepack. The node on PATH is used only when it has the
pinned major version and is at least NODE_MIN_VERSION, as
REPO_POLICIES.md asks. After installing either, bootstrap looks the tool
up through PATH again, stops naming the path if it is not the pinned
one, and prints the version it uses, as it does for Go. script/fmt and
script/fmt-check use the same node test, and otherwise run prettier
under the pinned node in nvm.

Model: opus-5-5

TODO.md

@@ -21,6 +21,17 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-19: `script/bootstrap` keeps the `yarn` on `PATH` only when
+  `yarn --version` is exactly `YARN_VERSION`, and otherwise prepares that
+  version through corepack, as it did when no yarn was installed
+  ([#154](https://git.eeqj.de/sneak/netwatch/issues/154)); any yarn used to be
+  kept. It uses the `node` on `PATH` only when that node has the major version
+  of `NODE_VERSION` as well as being at least `NODE_MIN_VERSION`, as
+  `REPO_POLICIES.md` asks; a node 24 was kept before. After installing node or
+  yarn it looks the tool up through `PATH` again and stops if that is not the
+  pinned one, as it does for Go, and it prints the node and yarn versions it
+  uses. `script/fmt` and `script/fmt-check` use the same node test, and
+  otherwise run prettier under the pinned node in nvm
 - 2026-10-19: `DEBUG` unset or empty leaves debug logging off, and any other
   value, `false` and `0` included, turns it on, as the code style guide says
   ([#159](https://git.eeqj.de/sneak/netwatch/issues/159)). No value of it stops