Fix CI startup failure: use only first-party actions

The apache org limits GitHub Actions to an allowlist; third-party
actions such as astral-sh/setup-uv are not on it and the run dies
immediately as a startup failure ("workflow file issue") without a
job ever starting. The registry and spec CI work the same way for the
same reason: they use actions/* only.

Replace astral-sh/setup-uv with actions/setup-python (first-party)
plus the documented uv install path (python -m pip install uv). The
rest of the workflow is unchanged; the step sequence was verified
locally (uv sync --group validate, trainer smoke run).

Apache-ai: Yes
Generated-by: pi (claude-sonnet-5-5)
Reviewed-by: anierbeck

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d008491..2c4a308 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -27,7 +27,11 @@
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: astral-sh/setup-uv@v5
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+      - name: Install uv
+        run: python -m pip install uv
       - name: Install dependencies
         run: uv sync --group validate
       - name: Train (tiny smoke settings)