fix: expose harbor package namespace to overlays

Adds harbor (self.packages) to overlay arguments, allowing custom
packages to reference each other via pkgs.harbor.

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

modules/flake/args.nix

@@ -24,6 +24,7 @@
             inherit lib;
             pkgs = prev;
           };
+          harbor = inputs.self.packages.${system} or {};
         })
       ];
     };