The dashboard LOAD tab always showed "no running agent containers":
container_stats::gather() looked for each agent's cgroup at
machine.slice/machine-h\x2d<name>.scope, but that path never exists.
nixos-container runs `systemd-nspawn --keep-unit` with
`Slice = "machine.slice"` (nixpkgs virtualisation/nixos-containers.nix),
so --keep-unit means no separate machined `machine-<name>.scope` is
created — the container's cgroup IS the launching service unit,
`container@<machine>.service`, under machine.slice. systemd-machined
still logs "New machine <name>" (registration), which is what made the
scope-path assumption look plausible, but the cgroup stays on the unit.
Fix scope_dir to machine.slice/container@<machine>.service and drop the
\x2d escaping (the service unit name is used verbatim in the cgroup dir;
the journal shows the literal `container@h-<agent>.service`). Adds a unit
test pinning the path.
6c3a83ffa1