Update README with npx usage instructions for Codex

[994AK]

Summary

Add documentation for running Codex via npx so that users can try the CLI without installing it globally.

Changes

• Update the top-level installation snippet to include:
  • npx @openai/codex@latest as an alternative to global installation.
• Extend the Quickstart section with a short example and explanation of running Codex via npx.

Motivation

Some users prefer to try Codex CLI without modifying their global environment or installing additional binaries. Providing an npx example:

• Lowers the friction for first-time users.
• Matches common patterns in other JavaScript/TypeScript tooling.
• Makes it clearer how to quickly run the latest version of Codex.

For CI or automation scenarios, the docs also encourage pinning a specific version instead of relying on @latest to avoid unexpected breakage from future releases.

Related issue

• Closes #

I have read the CLA Document and I hereby sign the CLA

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[994AK]

I have read the CLA Document and I hereby sign the CLA

[etraut-openai]

Thanks for the contribution, but I think we'll pass on this. We want to keep the installation instructions minimal and simple.