build: pin local uv Python to 3.13

requires-python is ">=3.13", so a fresh worktree's "uv sync --dev" picks the
newest interpreter on the box - currently CPython 3.15.0a7 - and the build
fails because pyo3 0.26 (via jiter/fastuuid/pydantic-core) does not support
3.15 yet:

    error: failed to run custom build command for `pyo3-ffi v0.26.0`

CI already pins 3.13 via setup-uv, so this only bit local worktrees. Pinning
matches CI and leaves requires-python alone.

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

.python-version

@@ -0,0 +1 @@
+3.13