Cleanup npm artifacts and consolidate on pnpm

[esthor]

The project seems to have been scaffolded with npm and pnpm and has artifacts from both.

This can cause significant dependency drift, inconsistent builds and repros, etc.

Since pnpm is preferred in the package.json of the project already, and it is generally preferable to npm in the modern NextJS community, I've opted to cleanup and remove the npm artifacts and references to npm in the README and AGENTS files.

[vercel[bot]]

@esthor is attempting to deploy a commit to the openai Team on Vercel.

A member of the Team first needs to authorize it.

[esthor]

I can also push up a preinstall script and an engines update to the package.json so that we don't wind up in this situation again, but I didn't want to overload this PR. I can follow-up after this is merged.

[nickspaargaren]

I approve this PR — it should be merged asap! 👍

In addition to the local install commands, we should also make sure the deployment install uses pnpm, since the site is hosted on Vercel:

https://vercel.com/docs/package-managers#deployment-override

// vercel.json

{
  "$schema": "https://openapi.vercel.sh/vercel.json",
  "installCommand": "pnpm install"
}

This could be done in a separate PR as well.