docs: define security fixer policy

[kamalsrini]

Summary

• Defines the Security Fixer Policy with auto-fix, assisted-fix, guidance-only, and human-review-required categories
• Adds hard human-review gates for high-risk remediation paths
• References the policy from nine fixer-capable skills
• Updates the skill template to require policy classification before applying changes

Linear

• UNI-13

Verification

• ruby scripts/validate_skill_schema.rb
• ruby scripts/validate_index.rb
• ruby scripts/test_skill_fixtures.rb
• fixer-policy reference count across 9 target skills
• git diff --check