feat: Add Backup/Restore Functionality

[nvandessel]

Add Backup/Restore Functionality

Overview

Add commands to backup current dotfiles state and restore from backups.

Acceptance Criteria

• Create timestamped backups
• List available backups
• Restore from specific backup
• Automatic backup before destructive operations (optional)
• Configurable backup location
• Backup compression support

Commands

g4d backup create           # Create backup
g4d backup list             # List backups
g4d backup restore <name>   # Restore backup
g4d backup delete <name>    # Delete backup

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Linear: https://linear.app/gargas/issue/G4D-22