Add undo command for reverting changes

[patmakesapps]

Problem

Every coding assistant makes mistakes. Users need a quick way to say "undo the last change" or "roll back to before that commit" without manually using git.

Proposed Solution

• undo command that reverts the last file change
• undo N to revert last N changes
• Uses git under the hood for safety

Acceptance Criteria

• undo reverts the most recent file modification
• undo N reverts the last N changes
• Works safely with git history
• Clear feedback on what was undone