docs: link theopenengine.com and zeroshot.sh from the README

[EivMeyer]

Summary

README readers had no path to a website. Two links change near the top of the README:

• The existing Zeroshot mention now links to zeroshot.sh instead of the Zeroshot repo (zeroshot.sh links to the repo itself).
• A short "Opcore is built by The Open Engine" sentence links theopenengine.com.

Both carry ?utm_source=github&utm_medium=readme&utm_campaign=opcore so site analytics can attribute the traffic. Documentation links and the npm homepage (https://the-open-engine.github.io/opcore/) are unchanged.

Checks

• scripts/build-docs.sh passes; the links render on the generated index.html
• opcore check --repo .: nothing to check (Markdown only)
• Rust checks / npm tests: not run, no Rust, package, or install change

Notes for reviewers

theopenengine.com/opcore answers 301 to zeroshot.sh/#opcore, a one-line section that links back to this repo, so the README does not use it and the npm homepage stays on the docs. The README is also the docs site's index page, so docs visitors who click these links are attributed to utm_medium=readme as well.

🤖 Generated with Claude Code