Link reviewer brief from README

[stacknil]

Summary

• add docs/reviewer-brief.md to the README Repo Guide
• assert the README keeps the reviewer brief entrypoint alongside the reviewer pack/path/architecture links

Validation

• pytest tests/test_reviewer_docs.py tests/test_markdown_links.py --basetemp .pytest-artifacts-reviewer-brief-link
• pytest --basetemp .pytest-artifacts-full-reviewer-brief-link
• git diff --check
• git diff privacy/secret keyword scan