Put "Codex Review" header in PR review text

[rolandwalker]

Description

Put "Codex Review" header in PR review text, since the source is not labeled other than "github-actions".

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

No blocking findings in this PR.

• The workflow change in .github/workflows/codex-review.yml is a presentation-only change (prefixing the posted comment with ## Codex Review:) and does not alter review selection, permissions, or execution logic.
• The changelog update in changelog.md is consistent with the workflow change.

Residual risk / gap:

• This behavior is in GitHub Actions YAML and is not covered by automated tests, so any regressions would be detected only when the workflow runs on an actual PR.