docs(sbom): clarify reviewer path

[stacknil]

Summary

• add a dedicated reviewer path for sbom-diff-and-risk
• route reviewers by question: orientation, artifacts, reproduction, release evidence, deep review
• update README entry points to make reviewer flow the first stop

Validation

• git diff --check
• python scripts/regenerate-example-artifacts.py --check
• python scripts/regenerate-example-artifacts.py --check --only requirements

Privacy / security

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