magrathea: --help for subcommands

I ran mg shell --help, and it wasn't very helpful:

❯ mg shell --help
[mg] entering shell for //tvix/store/--help
error: attribute '"--help"' missing for call to 'getAttr'

       at «string»:1:17:

            1| with builtins; (getAttr "--help" (getAttr "store" (getAttr "tvix" (import /home/flokli/dev/nixos/code.tvl.fyi-2 {}))))

I understand it might be too much work to get per-subcommand help right now, but could detect a --help in the args, and at print the generic usage info as long as there's no per-subcommand help.