week 1 module 3 add CommitManager skeleton and CLI commands

[DanisLol]

Summary

• Added 'CommitManager' skeleton
• Wired 'minigit commit', 'minigit branch', and 'minigit checkout'
• Added tests/test_commits.py with fake store and tree for testing

Validation

• scripts/quality-check.sh
• Manual testing, if applicable:

Checklist

• The change is focused and easy to review.
• Tests were added or updated for behavior changes.
• Documentation was updated for user-facing or setup changes.
• No secrets, generated artifacts, or local-only files are included.

Notes for Reviewers

• CommitManager() doesn't have default store so 'minigit commit' will throw an error. Will work when M1 has ObjectStore