Pin local Python to 3.13 (was 3.14)

Python 3.14 + uv kept producing partially-extracted wheels for
C-extension-heavy packages — protobuf and cachetools both manifested
the same symptom (dist-info present, package directory missing). The
runtime.txt and Dockerfile both pin 3.13 for HF Spaces; the local
.python-version was the only place still on 3.14, so dev and prod
disagreed.

Pinning local to 3.13 equalises dev/prod and removes a class of
'venv mysteriously broken after uv sync' issues. pyproject.toml's
requires-python = '>=3.13' floor is unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.python-version

@@ -1 +1 @@
-3.14
+3.13