feat: add fixer review and rollback guidance

[kamalsrini]

Summary

• Add fixer workflow review gates for change isolation, approval, reviewer evidence, and rollback guidance.
• Update the skill template to require review/rollback gates for fixer-capable workflows.
• Update fixer-capable AppSec, Cloud, and DevSecOps skills to include policy review gates, reviewer evidence, and rollback guidance.

Verification

• ruby scripts/validate_skill_schema.rb
• ruby scripts/validate_index.rb
• ruby scripts/test_skill_fixtures.rb
• git diff --check

Linear: UNI-16