feat: add checkov-security-scan Agent Skill

[alxayo]

• Add SKILL.md with comprehensive Checkov documentation

  • 1000+ security checks across 20+ IaC frameworks
  • Support for Terraform, CloudFormation, Kubernetes, Dockerfile, etc.
  • MITRE ATT&CK mappings for threat classification
  • Configuration options and CI/CD integration examples

• Add README.md with quick reference and usage examples

• Add examples/misconfiguration-patterns.md with:

  • Terraform (AWS) insecure patterns and fixes
  • Kubernetes security issues
  • Dockerfile best practices
  • GitHub Actions workflow security
  • CloudFormation examples
  • Common remediation patterns