Add GitHub Pull Request explanation

[calchiwo]

Add a flag to generate an explanation for a specific GitHub pull request.

Example:

explainthisrepo facebook/react --pr 27500

Implementation idea:

• Fetch the pull request metadata
• Fetch the PR diff and changed files
• Build a prompt describing the changes
• Generate an explanation of what the PR does

Relevant GitHub APIs:

GET /repos/{owner}/{repo}/pulls/{pull_number}
GET /repos/{owner}/{repo}/pulls/{pull_number}/files

The CLI should summarize the purpose of the PR and the main changes introduced.

Example output:

This pull request refactors the React scheduler to improve task prioritization and reduce unnecessary re-renders.....

This would make it easier to quickly understand what a pull request does without manually reading the entire diff.