refactor: move integration tests to tests/ directory

- Create tests/ directory for integration/manual test scripts
- Move test-mcp.mjs, test-real-video.mjs, test-bilibili.mjs to tests/
- Keep unit tests in src/__tests__/ (Jest convention)
- Update CHANGELOG.md

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 - Add Claude Code settings (.claude/, CLAUDE.md) to .gitignore
 - Add development guideline to always update CHANGELOG.md
+- Move integration test scripts to `tests/` directory for cleaner root
 
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