fix(ci): replace removed python/setup action with astral-sh/setup-uv

The custom python/setup composite action was removed during the
vergil-actions migration. Integration tests run on bare ubuntu-latest
(for Docker access) and need Python + uv installed explicitly, unlike
reusable workflow jobs that run inside vergil container images.

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

Author: wphillipmoore

.github/workflows/ci.yml

@@ -56,7 +56,7 @@ jobs:
         uses: actions/checkout@v6
 
       - name: Set up Python
-        uses: vergil-project/vergil-actions/actions/python/setup@v2.0
+        uses: astral-sh/setup-uv@v6
         with:
           python-version: ${{ matrix.python-version }}