docs: support AGENTS.md in review guidelines

[yudin-s]

Summary

• Update bundled code-review workflows to gather and cite AGENTS.md alongside CLAUDE.md
• Align feature-dev and pr-review-toolkit agents/docs to treat AGENTS.md as a project instruction source
• Define coexistence behavior for review guidance: apply both files, prefer CLAUDE.md when rules conflict

Fixes #6235

Test plan

• git diff --check
• git diff HEAD~1 --check