Install ESLint and prettier with yarn 4 through corepack (closes #493)

yarn 1, which the node image ships and which is no longer developed,
printed node's url.parse() deprecation warning during the js-deps
install. package.json now pins yarn 4.18.1 by version and hash in its
packageManager field; the js-deps stage enables it with the node
image's own corepack and runs `yarn install --immutable`. yarn.lock is
regenerated in yarn 4's format from the old lockfile, so every package
keeps the version it had. The new .yarnrc.yml keeps the install in
node_modules/, where the lint and Markdown stages run the tools from.

Model: opus-5-5

package.json

@@ -3,5 +3,6 @@
     "devDependencies": {
         "eslint": "10.11.0",
         "prettier": "3.9.9"
-    }
+    },
+    "packageManager": "yarn@4.18.1+sha512.b2e1e7524f654f2749d32b4ebcb4622473cb5bcbc485df2007e12a154e50162a4d795526768bc5f5b8f81717bfd79deb2472813d86fb5ae2eb551fa9c872b08f"
 }