hive-priv had no PATH, so the nixos-container it runs for every container
op (incl. lifecycle::list) failed ENOENT. build_all swallows that into an
empty list — the 'no managed containers' symptom. Give the helper the
minimal set nixos-container shells out to (nixos-container, nix, util-linux,
e2fsprogs) on top of the systemd/coreutils/findutils already in the unit
PATH, rather than the whole system profile.
Also route forge/matrix is_present() through priv_client::list_containers
instead of spawning nixos-container directly from unprivileged hive-c0re.
e7b6896e4c