[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 80.27%. Comparing base (bafcfa4) to head (6b34b0f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
- Coverage   84.64%   80.27%   -4.37%     
==========================================
  Files          15       15              
  Lines        1237     1237              
==========================================
- Hits         1047      993      -54     
- Misses        190      244      +54     

see 2 files with indirect coverage changes

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