[ADD][SSH] ssh-agent service using systemd

I needed to enable the service after linking the package. This might be
better served by yet another step during my Makefile-based installation?
I do not do it enough to warrant adding yet more steps for this
functionality.

Makefile

@@ -157,6 +157,11 @@ stow-scripts: scripts
 	mkdir -p $(STOW_TARGET)
 	$(STOW) $<
 
+stow-ssh: ssh
+	$(STOW) $<
+	# Enable & start ssh-agent service
+	systemctl enable --now --user ssh-agent.service
+
 stow-system-X: STOW_TARGET=/
 stow-system-X: system-X
 	sudo $(STOW) $<

ssh/.config/systemd/user/ssh-agent.service

@@ -0,0 +1,10 @@
+[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