CI: Add SHELL check

.github/workflows/lint-shell.yml

@@ -0,0 +1,33 @@
+---
+name: 🐶 SHELL check 🧪
+on:
+  push:
+    branches: '**'
+  pull_request:
+    branches: '**'
+  workflow_dispatch:
+
+jobs:
+  reviewdog:
+    permissions:
+      checks: write
+      contents: read
+      pull-requests: write
+    name: shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: haya14busa/action-cond@v1
+        id: reporter
+        with:
+          cond: ${{ github.event_name == 'pull_request' }}
+
+      - uses: reviewdog/action-shellcheck@v1
+        with:
+          reporter: ${{ steps.reporter.outputs.value }}
+          github_token: ${{ secrets.github_token }}
+          path: "."
+          check_all_files_with_shebangs: true
+          level: error
+          shellcheck_flags: '--external-sources --severity=error'