[codex] Add review guide pointer

[Marius1311]

Summary

• Add a shallow Review Guidelines section to AGENTS.md that points GitHub PR review agents to REVIEW_GUIDE.md.
• Keep review-specific workflow, risk areas, testing checks, and documentation-impact checks owned by REVIEW_GUIDE.md.

Validation

• Commit hooks passed.
• Not run: test suite, documentation-only change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.37%. Comparing base (ea38130) to head (e98b727).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #67   +/-   ##
=======================================
  Coverage   86.37%   86.37%           
=======================================
  Files          13       13           
  Lines        1387     1387           
=======================================
  Hits         1198     1198           
  Misses        189      189           

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