fix(ci): use uv venv + uv run instead of --system

The `--system` flag fails on modern uv because uv-managed Python
installations are marked EXTERNALLY-MANAGED (PEP 668), rejecting
system-wide pip installs. This caused exit code 2 in CI.

Switch to the standard uv workflow: `uv venv` to create an env,
`uv pip install` (no --system) to populate it, and `uv run` to
execute commands within it. Verified locally with a clean-slate
end-to-end run (venv → install .[test] → pytest → ruff → black).

Author: kYangLi

.github/workflows/ci.yaml

@@ -18,18 +18,18 @@ jobs:
         uses: astral-sh/setup-uv@v7
         with:
           version: "latest"
+          python-version: "3.13"
 
-      - name: Set up Python
-        run: uv python install 3.13
-
-      - name: Install package (dev)
-        run: uv pip install --system -e ".[dev]"
+      - name: Create venv and install package
+        run: |
+          uv venv
+          uv pip install -e ".[dev]"
 
       - name: ruff check
-        run: ruff check .
+        run: uv run ruff check .
 
       - name: black --check
-        run: black --check .
+        run: uv run black --check .
 
   test:
     name: Test (Python ${{ matrix.python-version }})
@@ -46,12 +46,12 @@ jobs:
         uses: astral-sh/setup-uv@v7
         with:
           version: "latest"
+          python-version: ${{ matrix.python-version }}
 
-      - name: Set up Python ${{ matrix.python-version }}
-        run: uv python install ${{ matrix.python-version }}
-
-      - name: "Install package (test deps: jax + e3nn-jax)"
-        run: uv pip install --system -e ".[test]"
+      - name: Create venv and install package
+        run: |
+          uv venv
+          uv pip install -e ".[test]"
 
       - name: Run pytest
-        run: pytest tests -q
+        run: uv run pytest tests -q