`nix.gc` run as root cannot prune generations under
`~/.local/state/nix/profiles` (NixOS/nix#8508), and `nix-gc-env` only
looks in `/nix/var/nix/profiles`. Home-manager generations therefore
accumulated since August 2024 and filled the disk.
`nh clean all` enumerates users and walks their per-user profiles too.
It replaces `nix.gc.automatic`, which cannot be enabled alongside it.