Refactor: cleaned up CLI structure and improved readability

[yash3567]

Summary

This pull request refactors the main CLI script to improve code readability, maintainability, and error handling. The logic and features remain the same, but the structure is cleaner and follows modern JavaScript best practices.

Changes Made

• Refactored CLI entry point using commander more effectively
• Replaced legacy code patterns with modern ES6+ syntax (e.g., destructuring, optional chaining)
• Improved error messages with better feedback using chalk
• Ensured random issue selection handles edge cases correctly
• Kept full compatibility with existing options:
  • -o, --open
  • -f, --first
  • -a, --auth <token>

Why This is Useful

• Easier to read and maintain the codebase
• Better developer experience for future contributors
• Prevents subtle bugs like invalid random index selection
• Clean separation of concerns between input, processing, and output

Additional Notes

• No changes to external behavior or package exports
• Fully backward-compatible
• CLI can still be run the same way: good-first-issue react etc.

Let me know if you'd like this broken into smaller commits, or if you'd prefer additional test coverage or file separation.