nixos-container has no kill verb. The KillContainer priv request was
calling nixos-container kill which always fails. Replace with:
machinectl kill <name> --signal=SIGKILL
which sends SIGKILL to all processes in the container — the correct
semantics for a forced shutdown (called after graceful stop has been
attempted). Add a machinectl_run helper alongside container_run so
callers stay consistent.