Run pre-commit in CI

On the same single matrix leg as the error reporter, since the style checks do
not depend on the interpreter. The hook environments are cached against the
hook config so they are not rebuilt on every run.

Author: ml31415

.github/workflows/ci.yaml

@@ -30,6 +30,19 @@ jobs:
           enable-cache: true
           cache-dependency-glob: pyproject.toml
 
+      # Style checks are independent of the interpreter, so run them on the same single
+      # leg as the error reporter instead of once per Python version.
+      - name: Cache pre-commit environments
+        if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.11'
+        uses: actions/cache@v6
+        with:
+          path: ~/.cache/pre-commit
+          key: ${{ runner.os }}-pre-commit-${{ hashFiles('.pre-commit-config.yaml') }}
+
+      - name: Run pre-commit
+        if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.11'
+        run: uvx pre-commit run --all-files
+
       - name: Install package and test dependencies
         run: uv sync --extra dev