Add ssh-agent service

nixos-config/home-manager.nix

@@ -54,6 +54,22 @@ in
       ./home-manager/fish.nix
     ];
 
+    systemd.user.services = {
+      service-name = {
+        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";
+        };
+      };
+    };
+
   services.mako.enable = true;
   };
 }