Add branch checkout restore and fast-forward merge

[DanisLol]

Summary

• switch_branch restores working tree
• HEAD stays on current branch
• fast-forward merge implemented

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

• Draft until Week 4 - M2: Restore files and index on checkout #23 merges
• Two tests failed since we need real checkout