test: reviewdog/action-shellcheck inline annotations demo

.github/demo-reviewdog-shellcheck.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+# Demo file for reviewdog/action-shellcheck PR-review test.
+# Intentionally broken — do not source or execute.
+#
+# Mix of severities so the level=error / fail_level=error gate can be
+# observed: only the SC1xxx parser error below should turn into a PR
+# review-comment when the new maintenance-lint-scripts-reviewdog.yml
+# workflow runs.
+
+# SC2086 — unquoted variable expansion (info)
+arg=$1
+echo $arg
+
+# SC2034 — assigned but never used (warning)
+unused="dead"
+
+# SC2046 — quote $() to prevent word splitting (warning)
+ls $(echo /tmp)
+
+# SC1073 — parser error: unmatched $( (error)
+broken=$(echo hello