Add stackbox down

[abhibongale]

Implement stackbox down to stop and clean up the environment. With fully containerized libvirt, cleanup is simple: stop containers and optionally remove volumes. VMs inside the libvirt container are automatically removed when the container is removed.

Containerized Benefit: No need to manually clean up host VMs - everything is in containers!

Implementation

@cli.command()
@click.option('--volumes', is_flag=True, help='Also remove volumes (deletes all data including VMs)')
@click.confirmation_option(prompt='Stop the environment?')
def down(volumes):
    """Stop and remove environment.
    
    With containerized libvirt, VMs are inside the libvirt container and are
    automatically removed when the container is removed. No manual VM cleanup needed!
    """
    config_dir = Path('.stackbox')
    
    if not config_dir.exists():
        click.echo("❌ No StackBox environment found in .stackbox/", err=True)
        sys.exit(1)
    
    orchestrator = compose.DockerComposeOrchestrator(config_dir)
    
    click.echo("🛑 Stopping containers...")
    orchestrator.down(remove_volumes=volumes)
    
    if volumes:
        click.echo("   Removed volumes (including VM disks)")
    
    click.echo("✅ Environment stopped")
    click.echo("\nTo restart: stackbox init ...")

Acceptance Criteria

• Stops all containers
• Removes containers
• --volumes removes data
• Deletes VMs
• Confirmation prompt works