Add reviewer start section

[stacknil]

Summary

• add a top-level Reviewer Start section to the README
• front-load links to the reviewer brief, reviewer path, reviewer pack, and docs index
• add reviewer-doc assertions so the start section remains stable

Validation

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