ci: enable Codex review bot on pull requests

[amjith]

Motivation

• Automate a first-pass code review using the Codex GitHub Action to provide concise, actionable feedback on pull requests.
• Ensure reviews run only for non-draft PRs and that feedback is posted back to the PR as a comment.
• Keep the workflow scoped and minimally privileged by checking out the PR merge commit and limiting contents permission to read.

Description

• Add a new workflow file at .github/workflows/codex-review.yml that triggers on pull_request_target events (opened, reopened, synchronize, ready_for_review).
• The workflow checks out the PR merge ref, pre-fetches base/head refs, then runs openai/codex-action@v1 with a prompt that focuses review on the PR diff and common concerns.
• The workflow captures Codex output and a follow-up job posts the result as a PR comment using actions/github-script, with the expectation that OPENAI_API_KEY is configured in repository secrets.
• Add an Unreleased entry to CHANGELOG.md under Internal documenting the new workflow.

Testing

• Ran git diff --check to validate no whitespace or diff issues and the check passed.

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Codex Task