chore(internal): add contributors update skill

[julienld]

Summary

• add a /contributors-update skill to identify merged PR authors missing from the README
• require a preview and explicit approval before editing or pushing
• allow approved documentation-only (*.md) adjustments to be pushed directly to master without a PR
• require a clean, current master, an exact diff preview, and explicit approval for both the change and direct push
• keep code, configuration, generated artifacts, and workflow files on the normal worktree and draft-PR path
• document the new skill in AGENTS.md

Testing

• git diff --check
• documentation-only change; no runtime tests required

🤖 Generated with Claude Code

[ghhamcp]

@codex review — apply the review criteria in .gemini/styleguide.md in addition to AGENTS.md guidance

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: d3f8e9c3be

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[julienld]

✅ Addressed all five Codex review findings in cef4981: primary-checkout worktree discovery, merged-history marker lookup, server-side merge-date filtering, untrusted PR metadata handling, and post-PR CI/review validation.