Fix rubocop configuration not being loaded

rubocop 1.86.0 prioritizes ~/.config/rubocop/config.yml over the
project's .rubocop.yml, causing all project settings to be ignored.
Explicitly specify the config file in the Rake task to work around this.

Also update rubocop-performance from require to plugins directive,
as required by rubocop-performance 1.24+.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: hsbt

.rubocop.yml

@@ -1,5 +1,5 @@
 inherit_from: .rubocop-bundler.yml
-require:
+plugins:
   - rubocop-performance
 AllCops:
   NewCops: disable

Rakefile

@@ -4,7 +4,11 @@ require "bundler/gem_tasks"
 require "rspec/core/rake_task"
 require "rubocop/rake_task"
 
-RuboCop::RakeTask.new
+# Workaround: rubocop 1.86.0 prioritizes ~/.config/rubocop/config.yml
+# over the project's .rubocop.yml, so we explicitly specify the config file.
+RuboCop::RakeTask.new do |t|
+  t.options = %w[-c .rubocop.yml]
+end
 
 desc "Run specs"
 RSpec::Core::RakeTask.new(:spec) do |t|