services.ssh-agent for hm,
programs.ssh.startAgent for nixos. Both have some other fun
stuff in programs.ssh that I haven't really figured out
yet. Also the config is complete bullshit, more on that in hm-vs-nixos.
ssh-add <id> to add an identity, other flags
control the storage. ssh-agent -t 1h might be good, but
that depends on my chosen workflow. Also hm doesn't have this, might
want to patch it in.