Codex code review cookbook updated with cross-CI/CD support and GPT-5.5

AI Impact Summary

This update expands Codex-assisted code reviews from the cloud-hosted flow to on-prem or alternative CI/CD environments by documenting a headless Codex CLI workflow that runs in GitHub Actions, Jenkins, GitLab CI/CD, and Azure DevOps Pipelines. It also shifts the recommended model toward GPT-5.5 for code-review accuracy and includes safety considerations around API key isolation in CI contexts. Operators should plan migration to the GPT-5.5 workflow, ensure secrets governance for PR review runs, and adapt pipelines to emit structured findings back into PR threads.