Enable daily auto-upgrade on strixi-minaj, railbird-sf, and ryzen-shine

Set the autoUpgrade flake reference globally in configuration.nix so
machines only need to opt in with system.autoUpgrade.enable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nixos/machines/railbird-sf.nix

@@ -90,5 +90,10 @@
     }
   ];
 
+  system.autoUpgrade = {
+    enable = true;
+    dates = "daily";
+  };
+
   system.stateVersion = "23.11";
 }