ssh-agent.el

  (defun ssh-reagent ()
    (interactive)
    (let* ((sshdir (car (directory-files "/tmp" nil "ssh-*")))
	   (agent (car (directory-files (concat "/tmp/" sshdir) nil "agent.*"))))
      (setenv "SSH_AUTH_SOCK" (concat "/tmp/" sshdir "/" agent)))
    (message "Attached to SSH Session"))

ssh-agent.go

func GetSSHAgentPath() (out string, err error) {
	usr, err := user.Current()
	if err != nil {
		return "", err
	}

	if path := filepath.Join("/run/user", usr.Uid, "ssh-agent-socket"); FileExists(path) {
		return path, nil
	}

	err = filepath.Walk("/tmp", func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return nil
		}

		if info.IsDir() {
			return nil
		}
		if !strings.HasPrefix(path, "/tmp/ssh-") {
			return nil
		}

		out = path
		return io.EOF
	})

	if out == "" || err == nil {
		return "", errors.New("could not find ssh socket")
	}
	return out, nil
}

ssh-agent.service

[Unit]
Description=SSH key agent

[Service]
Type=simple
Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket
ExecStart=/usr/bin/ssh-agent -D -a $SSH_AUTH_SOCK

[Install]
WantedBy=default.target

ssh-agent.sh

ssh-reagent () {
        for agent in /tmp/ssh-*/agent.*
        do
                export SSH_AUTH_SOCK=$agent
                if ssh-add -l 2>&1 > /dev/null
                then
                        echo Found working SSH Agent:
                        ssh-add -l
                        return
                fi
        done
        echo Cannot find ssh agent - maybe you should reconnect and forward it?
}