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/configs.nix

@@ -1,26 +1,38 @@
-{ config, pkgs, ... }:
+{ config, pkgs, lib, ... }:
 
 let
   configDir = ../../config;
   
-  # Read the contents of the config directory
+  # Read the contents of the upstream config directory
   configEntries = builtins.readDir configDir;
   
+  # Check for user overrides
+  userConfigDir = config.nomarchy.configOverrides;
+  userEntries = if userConfigDir != null && builtins.pathExists userConfigDir
+                then builtins.readDir userConfigDir
+                else {};
+
   # Files to explicitly exclude (handled elsewhere or not intended for ~/.config/)
   excludedFiles = [
     "nomarchy.ttf"
   ];
   
+  # Get all unique names from both sources
+  allNames = lib.unique (builtins.attrNames configEntries ++ builtins.attrNames userEntries);
+  
   # Filter the entries
-  validEntries = builtins.filter (name: !(builtins.elem name excludedFiles)) (builtins.attrNames configEntries);
+  validEntries = builtins.filter (name: !(builtins.elem name excludedFiles)) allNames;
 
   # Generate the xdg.configFile attribute set
+  # If a name exists in userEntries, it takes precedence.
   # For directories, we use `recursive = true;` to allow the user to create their own files alongside the read-only defaults.
   makeMapping = name: {
     inherit name;
-    value = {
-      source = "${configDir}/${name}";
-      recursive = configEntries.${name} == "directory";
+    value = lib.mkDefault {
+      source = if userEntries ? ${name} 
+               then "${userConfigDir}/${name}" 
+               else "${configDir}/${name}";
+      recursive = (userEntries.${name} or configEntries.${name}) == "directory";
     };
   };
 

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.";
+    };
   };
 }