feat: add configurable GitLab URL support via GITLAB_URL environment variable
- Add internal/config package for centralized configuration management
- Add internal/gitlabclient package with URL validation and security checks
- Enforce HTTPS for custom GitLab URLs to prevent token leakage
- Add graceful shutdown handling with signal.NotifyContext
- Refactor main.go for better separation of concerns and maintainability
- Support both default gitlab.com and custom GitLab instances
Co-Authored-By: Claude noreply@anthropic.com