Improve reviewer authoring and version governance

[bhdicaire]

Summary

• add a copyable reviewer scaffold package under reviewer-scaffold/example-reviewer
• add validate_reviewer_contract.py for structural reviewer package checks
• document when reviewer versions should and should not be bumped
• update the reviewer blueprint frontmatter example to include version
• add contract tests for the scaffold, validator, and version-governance docs

Tests

• python3 -m unittest discover -s tests -p 'test_*.py'
• python3 skills/wgo/scripts/validate_reviewer_contract.py skills/wgo/references/reviewer-scaffold/example-reviewer skills/wgo/references/reviewers/* --core-id architecture --core-id business-continuity --core-id code-quality --core-id contributor-vendor-value --core-id expense-exposure --core-id maintenance-cost --core-id product-value --core-id project-health --core-id revenue-risk --core-id scalability --core-id security-privacy