Add ssh-agent service and ssh config

Add a systemd user service to start the ssh-agent and add an ssh
config to automatically add ssh identities to the agent when they
are used (otherwise we have to add them manually). I will see how
this works out.

See: https://wiki.archlinux.org/title/SSH_keys#SSH_agents

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

@@ -0,0 +1,13 @@
+# From: https://wiki.archlinux.org/title/SSH_keys#SSH_agents
+[Unit]
+Description=SSH key agent
+
+[Service]
+Type=simple
+Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket
+# DISPLAY required for ssh-askpass to work
+Environment=DISPLAY=:0
+ExecStart=/usr/bin/ssh-agent -D -a $SSH_AUTH_SOCK
+
+[Install]
+WantedBy=default.target

dotfiles/ssh/config

@@ -0,0 +1 @@
+AddKeysToAgent yes