Exclude demo folder from SonarCloud analysis

Add demo/** to sonar.exclusions to prevent false positive warnings
about code duplication in example/demonstration code.

The demo folder contains sample code that intentionally shows similar
patterns across multiple files for educational purposes. Code duplication
in demos is expected and not a quality issue.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: koriym

.sonarcloud.properties

@@ -1,2 +1,4 @@
-# Exclude GitHub workflow files from security hotspot analysis
-sonar.exclusions=.github/workflows/**
+# Exclude files from SonarCloud analysis
+# - GitHub workflow files: version tags are best practice
+# - Demo files: example code with intentional duplication for demonstration
+sonar.exclusions=.github/workflows/**,demo/**