chore: Add .node-version file and update workflows to use it

[baruchiro]

Added .node-version file and updated GitHub Actions workflows to use it for Node.js version management.

[cliffhall]

Hi @baruchiro!

This seems nice, but I'd much rather use package.json for the file as opposed to adding yet another file to the root of the project for a single variable.

Docs for setup-node show how you can do this. (note, using setup-node@v5):

In main.yml

      - uses: actions/setup-node@v5
        with:
          node-version-file: package.json
          cache: npm

and in package.json

{
  "engines": {
    "node": ">=22.0.0"
  }
}