Update PR Summary

Update a PR title and description based on the complete changeset.

When explicitly invoked with extra text, treat that text as the PR number or URL.

Process

1. Fetch PR info

   • Use gh pr view <pr> --json title,body,baseRefName,headRefName.

2. Analyze the complete changeset

   • Use git diff <base-branch>...HEAD to review all committed changes in the branch.
   • Ignore unstaged changes.
   • Make the summary describe the full branch diff, not just the latest commit.