Fix RuboCop to inherit default Exclude patterns

- Add `inherit_mode: { merge: [Exclude] }` to inherit default exclusion patterns
- Remove explicit `vendor/**/*` exclusion (now inherited from default config)
- Remove obsolete `gemfiles/**/*` exclusion (gemfiles directory was removed)

This ensures that `vendor/bundle` is properly excluded when using
`bundler-cache: true` in GitHub Actions workflow, preventing RuboCop
from inspecting gem files and loading their .rubocop.yml configurations.

Fixes the issue where RuboCop tried to require 'rubocop-rake' from
gems installed in vendor/bundle.

Author: snaka

.rubocop.yml

@@ -1,10 +1,11 @@
+inherit_mode:
+  merge:
+    - Exclude
+
 AllCops:
   TargetRubyVersion: 2.5
   DisabledByDefault: true
   SuggestExtensions: false
-  Exclude:
-    - 'gemfiles/**/*'
-    - 'vendor/**/*'
 
 Style/AndOr:
   Enabled: true