chore(scripts): check if install is run with npm or yarn

Author: caugner

npm-check.js

@@ -0,0 +1,18 @@
+if (process.env.npm_config_user_agent?.includes("yarn")) {
+  console.error();
+  console.error(
+    "❌ Yarn is no longer supported in this repository (since December 1, 2025).",
+  );
+  console.error("   Use npm instead:");
+  console.error("   - `npm install`     instead of `yarn install`");
+  console.error("   - `npm run content` instead of `yarn content`");
+  console.error();
+  process.exit(1);
+} else {
+  console.info();
+  console.info("🐥 This repository is now using npm (since December 1, 2025).");
+  console.info(
+    "🐞 Please report any issues here: https://github.com/mdn/fred/issues/new?template=bug.yml",
+  );
+  console.info();
+}

package.json

@@ -37,6 +37,7 @@
     "lint:typos-group-by-file": "cspell --no-progress --gitignore --quiet --reporter cspell-group-by-file-reporter --config .vscode/cspell.json \"**/*.md\"",
     "lint:typos-words-only": "cspell --no-progress --gitignore --words-only --unique --config .vscode/cspell.json \"**/*.md\"",
     "lint:yml": "prettier -c \"**/*.yml\"",
+    "prepare": "node npm-check.js",
     "start": "npm run --silent info:fred && npm run up-to-date-check && env-cmd --silent cross-env CONTENT_ROOT=files NODE_ENV=production FRED_PLAYGROUND_LOCAL=true FRED_PLAYGROUND_PORT=5043 FRED_PORT=5042 FRED_WRITER_MODE=true fred-server",
     "test": "node --test",
     "up-to-date-check": "node scripts/up-to-date-check.js"