Add remediation regression fixture tests

[kamalsrini]

Summary

• Extend the fixture harness with optional remediation regression blocks and expected unified-diff checks.
• Add a remediation-only wrapper command for auto-fix candidate checks.
• Add expected remediation files for secure-code-review SQL parameterization and secrets-management hardcoded secret removal.

Verification

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

Linear: UNI-11