chore: Pin the Node version with .nvmrc and an engines field

Summary The README requires Node 20+, but nothing enforces it. Adding .nvmrc and an engines field gives contributors a one-command way to use the right version.

Where

• New file .nvmrc at repo root.
• Root package.json.

Steps

1. Branch: git checkout -b chore/pin-node-version.
2. Create .nvmrc with a single line: 20.
3. In root package.json add:

   "engines": { "node": ">=20" }
   

4. Add one line to README/CONTRIBUTE prerequisites: "Run nvm use to match the project's Node version."
5. Run yarn install then yarn lint.
6. Commit chore: pin node version via .nvmrc and engines and open a PR.

Acceptance criteria

• .nvmrc exists with 20.
• Root package.json has engines.node.
• nvm use selects Node 20.

Good first issue — comment here if you'd like to take it.