docs: clarify repository reviewer entry

[stacknil]

Summary

• make the root reviewer brief a clear routing page for repository-level review
• link the SBOM reviewer path, reproduction guide, verification guide, and supporting mini-lab entry point
• add the root reviewer brief to the README's reviewer-facing entry points

Validation

• git diff --check
• python scripts/regenerate-example-artifacts.py --check
• checked referenced local paths exist

Privacy / security

• checked changed docs for local paths, school identifiers, and secret-like strings