smolvm pack create --from-vm requires the VM to be stopped, and stopping
a smolmachines VM terminates any running interactive session.
Instead, mirror the macos-container approach: exec into the running VM as
root and stream the root filesystem via tar (smolvm machine exec -- tar),
build a Docker image from the archive, push to an ephemeral local registry,
and run smolvm pack create --image to produce the .smolmachine artifact.
The VM stays running throughout the commit.
Remove the stop-confirm prompt and machine_is_running check that were
added in the previous commit — neither is needed when we no longer stop.
didericis-claude
921aceb515