Add REVIEW_GUIDE.md as the repository's canonical code-review guide, distilled
from the maintainer's review history. Centralize the review framework: move the
AI Review Guidance out of AGENTS.md into the guide, move the engineering
conventions into AGENTS.md's Code Style section, and update the CodeRabbit and
Qodo configs to apply the guide and split review focus between the two bots
(CodeRabbit on correctness/conventions, Qodo on security/performance).
