Publish Python SDK to PyPI

Make the Python SDK installable via pip install knowledgemesh.

**Includes:** KM class with chat(), whoami(), models(), recover(), reset_secret(), completions_create()
**OpenAI compat:** completions_create() works as drop-in
**Error handling:** KMError with proper non-JSON response handling

## Reference
See `old_mvp/km/sdk/python/`.