Fix Vulnerability PR

Process a Snyk (or similar) dependency upgrade PR: rename the PR, regenerate the lockfile, analyze changelogs for breaking changes, and post a summary comment.

The user must provide a PR URL or number as $ARGUMENTS. If not provided, ask for it.

Step 1: Fetch PR details

Extract the PR number from the argument and fetch the PR diff to understand what changed:

# Get PR metadata
gh pr view <PR_NUMBER> --json number,title,body,headRefName,baseRefName

# Get the diff to see which dependencies changed
gh pr diff <PR_NUMBER>