Split PR by CODEOWNERS Groups

Split a large pull request into multiple smaller PRs, where each PR touches the fewest possible CODEOWNERS reviewer groups. The goal is to reduce review burden: a PR that only touches megatron/core/ needs only the core reviewers, while a PR that also touches examples/, tools/, and megatron/training/ pulls in many additional groups.

Workflow

1. Analyze the PR

1. Fetch the PR details: gh pr view <number> --repo NVIDIA/Megatron-LM --json title,body,headRefName,author and gh pr diff <number> --repo NVIDIA/Megatron-LM --stat. Also determine the current GitHub user with gh api user --jq .login.
2. Parse .github/CODEOWNERS to build a mapping from file path patterns to owner groups.
3. For each changed file in the PR, determine which CODEOWNERS groups would be required to review it.
4. Build a summary table grouped by CODEOWNERS group, showing which files pull in which groups.
5. Count the total number of distinct reviewer groups the PR currently requires.

2. Propose a split that minimizes reviewer groups per PR