fix: resolve evaluation purity, missing packages, and brittle paths

core/home/overrides.nix

@@ -20,14 +20,9 @@
 let
   overridesDir = "${config.home.homeDirectory}/.config/nomarchy/overrides";
 
-  # Check if a specific override exists
-  hasOverride = path: builtins.pathExists "${overridesDir}/${path}";
-
-  # Get override source if it exists, otherwise use default
+  # Helper to get override from options
   getOverrideOrDefault = { path, default }:
-    if hasOverride path
-    then "${overridesDir}/${path}"
-    else default;
+    config.nomarchy.overrides.paths.${path} or default;
 
 in
 {