Only apply ZeitwerkFriendlyConstant cop to app directory by default

Author: shioyama

config/default.yml

@@ -1223,6 +1223,8 @@ Rails/ZeitwerkFriendlyConstant:
   Description: 'Ensure all constants defined in each file are independently loadable by Zeitwerk.'
   Enabled: 'pending'
   VersionAdded: '<<next>>'
+  Include:
+    - app/**/*.rb
 
 # Accept `redirect_to(...) and return` and similar cases.
 Style/AndOr: