feat(hydra): build systemConfigs as hosts jobs

Fold outputs.systemConfigs into the hydra 'hosts' jobs alongside
nixosConfigurations, so Hydra builds mgc's system-manager generation as
hosts.mgc (matching the hosts.<name> convention the auto-upgrade uses).

Assisted-by: pi (claude-opus-4-8)

hydra.nix

@@ -16,6 +16,8 @@
     pkgs;
 in {
   pkgs = lib.mapAttrs filterValidPkgs outputs.packages;
-  hosts = lib.mapAttrs (_: cfg: cfg.config.system.build.toplevel) outputs.nixosConfigurations;
+  hosts =
+    lib.mapAttrs (_: cfg: cfg.config.system.build.toplevel) outputs.nixosConfigurations
+    // outputs.systemConfigs;
   homes = lib.mapAttrs (_: cfg: cfg.activationPackage) outputs.homeConfigurations;
 }