Add RuboCop workflow

.gitattributes

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+.rubocop_todo.yml linguist-generated

.github/workflows/rubocop.yml

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+name: RuboCop
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: RuboCop
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: '2.6.10'
+        bundler-cache: false
+    - name: RuboCop
+      run: |
+        gem install rubocop
+        rubocop

.rubocop.yml

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+inherit_from: .rubocop_todo.yml
+
+AllCops:
+  NewCops: enable
+  TargetRubyVersion: 2.6
+  Exclude:
+    - .git/**/*
+    - .github/**/*
+    - gemfiles/**/*
+    - node_modules/**/*
+    - tmp/**/*
+    - vendor/**/*
+
+Layout/LineLength:
+  Enabled: false