How to start ssh-agent as systemd service? There are some suggestions in the net, but they are not complete.How to add automatically unencrypted keys if ssh-agent service was started successfully? Probably, adding keys from the list of ~/.ssh/.session-keys would be good.How to set SSH_AUTH_SOCK in any login session afterwards? The most correct way is to push it from ssh-agent service to systemd-logind service (have no idea if it's ever possible). The plain naive way is just add it to /etc/profile.
How to start and use ssh-agent as systemd service?
systemd;ssh agent