feat: enhance repository with professional developer experience

✨ Added professional repository features:
- MIT License for open source compliance
- Comprehensive CONTRIBUTING.md with development guidelines
- GitHub issue templates (bug reports, feature requests, documentation)
- GitHub Actions workflow for automated OpenAPI validation
- CHANGELOG.md for tracking specification updates
- SECURITY.md with security best practices and disclosure policy
- Dependabot configuration for automated security updates

🛠️ Developer experience improvements:
- Structured issue reporting with proper templates
- Automated validation on every PR
- Security scanning and dependency management
- Clear contribution guidelines with testing requirements
- Professional documentation standards

🔐 Security enhancements:
- Environment variable best practices
- No hardcoded credentials policy
- Automated secret scanning
- Vulnerability disclosure process

This makes the repository production-ready for community contributions
while maintaining high standards for API specification accuracy.