feat(hydra): build themes packages

Add themes to hydraJobs by reusing the themes flake's own hydraJobs output
(inputs.themes.hydraJobs), so the sub-flake declares what it wants built.
Surfaces allColorschemes and allWallpapers per system.

Assisted-by: pi (opus-4.8)

hydra.nix

@@ -16,6 +16,7 @@
     pkgs;
 in {
   pkgs = lib.mapAttrs filterValidPkgs outputs.packages;
+  themes = inputs.themes.hydraJobs;
   hosts = lib.mapAttrs (_: cfg: cfg.config.system.build.toplevel) outputs.nixosConfigurations;
   homes = lib.mapAttrs (_: cfg: cfg.activationPackage) outputs.homeConfigurations;
 }