fix: move test artifacts to tmp directory and update gitignore

- Create tmp/test_artifacts/ directory for test storage
- Add tmp/ to .gitignore to exclude test artifacts from version control
- Update test files to use project tmp directory instead of system temp
- Add test-specific path constants for consistent configuration
- Prevent asset_registry.json from being overwritten by tests

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

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

markitect/assets/constants.py

@@ -10,6 +10,10 @@ DEFAULT_ASSETS_DIR = "assets"
 DEFAULT_REGISTRY_FILENAME = "asset_registry.json"
 DEFAULT_MANIFEST_FILENAME = "manifest.json"
 
+# Test-specific paths (for development/testing)
+DEFAULT_TEST_ASSETS_DIR = "tmp/test_artifacts/assets"
+DEFAULT_TEST_REGISTRY_FILENAME = "tmp/test_artifacts/asset_registry.json"
+
 # Package file extension
 PACKAGE_EXTENSION = ".mdpkg"