feat(downstream): add easy configuration overrides

- Introduce nomarchy.configOverrides option to map a user directory to ~/.config
- Implement automatic merging of upstream defaults and user overrides
- Use lib.mkDefault for all upstream mappings to allow granular HM overrides
- Update installer template with usage examples

modules/home/options.nix

@@ -73,5 +73,10 @@
         description = "System monospace font.";
       };
     };
+    configOverrides = lib.mkOption {
+      type = lib.types.nullOr lib.types.path;
+      default = null;
+      description = "Path to a directory containing configuration overrides.";
+    };
   };
 }