Add GitHub issue explanation

[calchiwo]

Add a flag to generate an explanation for a specific GitHub issue.
Example:

explainthisrepo facebook/react --issue 27500

Implementation idea:

• Fetch the issue metadata
• Fetch the issue comments
• Build a prompt combining the issue description and discussion
• Generate a summary explaining the problem and context
• Relevant GitHub APIs:

GET /repos/{owner}/{repo}/issues/{issue_number}
GET /repos/{owner}/{repo}/issues/{issue_number}/comments

The CLI should summarize the main problem being discussed and the key points from the conversation.
Example output:

This issue discusses a memory leak in React Suspense caused by improper cleanup during component unmounting. The discussion explores possible fixes and related edge cases....

This would make it easier to quickly understand issues and discussions in large repositories without reading long threads.