Add changes summary to CreatePRTool

[codegen-sh]

This PR enhances the create_pr function to show a summary of the changes included in the PR.

Changes Made

1. Added a new changes_summary field to the CreatePRObservation class
2. Created a generate_changes_summary function that parses git diffs to create a human-readable summary
3. Updated the create_pr function to generate and include the summary in the response

Summary Format

The changes summary includes:

• Number of files changed
• Number of lines added/deleted
• List of modified files grouped by file extension

This makes it easier for users to understand what changes are included in the PR without having to click through to GitHub.

Closes CG-1234