[FR] Add .nvmrc and .npmrc engine-strict so Node/pnpm versions auto-match

Use case

Onboarding a new contributor: they install some Node version, some pnpm version, hit pnpm install, and hope for the best. CONTRIBUTING.md prerequisites drift from package.json (see companion bug on pnpm 8 vs pnpm 10 pin).

Proposal

Add two small files at repo root:

1. .nvmrc with the target Node version (e.g. 20) so nvm use / fnm use / volta pick the right one automatically.
2. .npmrc with engine-strict=true so mismatched Node/pnpm errors early instead of silently misbehaving.

Optionally add a packageManager engines field enforcement via only-allow.

Existing art

Cloudflare's own workers-sdk ships .nvmrc; Vite, Next.js, Astro all do the same.

Not a bug

Repo works today; this is DX polish, not a correctness fix.

Thanks for maintaining idosal/git-mcp!