Show the differences between a checkpoint and the current workspace.

Displays what has changed since the checkpoint was created, including:
- Added, modified, and deleted files
- Lines added and removed
- Detailed file diff (when file parameter is provided)

Parameters:
- checkpoint_id: The checkpoint ID or commit hash to compare (default: most recent)
- file: Specific file path to show detailed diff (optional)

Use this to preview changes before restoring to a checkpoint.
