docs: fix package names and stale paths

- client-py is `donglora` on PyPI (not `donglora-python`)
- mux-py is `donglora-mux` on PyPI
- Fix client library links (clients/python → client-py, clients/rust → client-rs)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CONTRIBUTING.md

@@ -22,7 +22,7 @@ common tasks consume. No shared board code — each board is self-contained.
 
 ## Client Libraries
 
-Client libraries exist in [Python](clients/python/) and [Rust](clients/rust/).
+Client libraries exist in [Python](https://github.com/donglora/client-py) and [Rust](https://github.com/donglora/client-rs).
 To add a library in another language:
 
 1. Create a directory with the language's standard project layout