feat(tart-agent-sandbox): add sway desktop with auto-login

- Enable desktop role with wayland/sway
- Use greetd for passwordless auto-login to sway
- Add video/input groups to agent user

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

machines/tart-agent-sandbox/configuration.nix

@@ -13,6 +13,23 @@
 
   networking.hostName = "tart-agent-sandbox";
 
+  # Enable sway desktop
+  roles.desktop = {
+    enable = true;
+    wayland = true;
+  };
+
+  # Auto-login to sway (no display manager)
+  services.greetd = {
+    enable = true;
+    settings = {
+      default_session = {
+        command = "${pkgs.sway}/bin/sway";
+        user = "agent";
+      };
+    };
+  };
+
   # SSH access from host
   services.openssh = {
     enable = true;
@@ -26,7 +43,7 @@
   users.users.agent = {
     isNormalUser = true;
     description = "Agent sandbox user";
-    extraGroups = [ "wheel" "docker" ];
+    extraGroups = [ "wheel" "docker" "video" "input" ];
     initialPassword = "agent";
     openssh.authorizedKeys.keys = [
       # Add your SSH public key here for passwordless access