Merged pure doc PR without landpr

Greptile Summary

This PR updates CONTRIBUTING.md to require contributors with Codex access to run codex review --base origin/main locally before opening or updating a PR, and clarifies expectations around handling Codex review findings.

The changes include:

• Adds a codex review pre-PR step to the "Before You PR" checklist (line 89)
• Adds two bullets to "Review Conversations Are Author-Owned" covering how to handle Codex comments and what to do when GitHub Codex review doesn't trigger (lines 103–104)
• Adds a codex review checklist item to the "AI/Vibe-Coded PRs Welcome!" section (line 133)

The changes are internally consistent, accurately reflect the PR description, and are well-placed within the existing document structure with no broken links or contradictions.

Confidence Score: 5/5

• Docs-only change with clear, consistent additions to contribution guidelines; safe to merge.
• This is a docs-only change with no code modifications. All additions are clearly written, properly placed within existing sections, and align with the stated intent. No syntax errors, broken links, or contradictions with other sections. The three new requirements (local codex review before PR, handling Codex findings, and fallback to local review) are well-articulated and consistent throughout the document.
• No files require special attention.

_{Last reviewed commit: ee95a42}