fix: exclude non-Python dirs from setuptools package discovery

proxy/, assets/, references/ are not Python packages.
Explicit exclusion prevents flat-layout discovery error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: aeromomo

pyproject.toml

@@ -58,3 +58,6 @@ dev = ["pytest>=7.0"]
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
+
+[tool.setuptools.packages.find]
+exclude = ["proxy*", "assets*", "references*", "docs*"]