Track Homebrew's clang-format version instead of a fixed pin

The previous commit pinned CI and the pre-commit hook to a fixed
clang-format 23.1.0, but the maintainer develops on macOS against
whatever version `brew` currently installs, which changes over time
as Homebrew updates the formula.

style-check now runs on macos-latest and installs clang-format via
`brew install`, so it tracks the same moving target the maintainer's
Mac does. The pre-commit hook switches from pre-commit's own pinned
mirror to a local hook that shells out to the system clang-format,
so a local commit and CI both go through the same Homebrew-installed
binary rather than two independently versioned copies.

Trade-off: this reintroduces the non-determinism a fixed pin avoids
-- a commit's style-check result can now change over time as Homebrew
updates the formula -- but that mirrors how the maintainer already
develops, which is the point.

Also install coreutils in CI: the style_check Makefile target needs
grealpath's --relative-to, which the macOS-native realpath lacks.

.github/workflows/test.yaml

@@ -31,7 +31,10 @@ env:
 
 jobs:
   style-check:
-    runs-on: ubuntu-latest
+    # Uses the macOS runner's pre-installed Homebrew so clang-format tracks
+    # whatever version `brew install clang-format` currently resolves to on
+    # the maintainer's own Mac, instead of a version pinned in this file.
+    runs-on: macos-latest
     if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
     continue-on-error: true
     steps:
@@ -39,8 +42,8 @@ jobs:
         uses: actions/checkout@v4
       - name: install clang-format
         run: |
-          pipx install clang-format==23.1.0
-          echo "$HOME/.local/bin" >> "$GITHUB_PATH"
+          brew update
+          brew install clang-format coreutils
       - name: run style check
         run: |
           clang-format --version