Add repository comparison command

[calchiwo]

Add a command to generate a comparison between two repositories.

Example:

explainthisrepo compare facebook/react preactjs/preact

Implementation idea:

1. Run the existing repository analysis pipeline for both repositories

2. Generate explanations for each repository

3. Build a prompt that compares the two results

4. Produce a summary describing the key differences

Example output:

React uses a virtual DOM diffing approach and focuses on a full-featured ecosystem.

Preact provides a lighter implementation with a smaller bundle size while maintaining a React-compatible API.

This would help developers quickly understand differences between similar projects, frameworks, or libraries.