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.

.pre-commit-config.yaml

@@ -1,7 +1,12 @@
 repos:
-  - repo: https://github.com/pre-commit/mirrors-clang-format
-    rev: v23.1.0  # CIのstyle-checkと同じバージョンに揃える
+  - repo: local
     hooks:
       - id: clang-format
+        name: clang-format
+        # Uses whatever `clang-format` resolves to on PATH (the Homebrew
+        # install on macOS) instead of a version pinned here, so it tracks
+        # the same version CI installs via `brew install clang-format`.
+        entry: clang-format
+        language: system
         files: ^(httplib\.h|(example|fuzzing|test)/[^/]+\.(cpp|cc|h))$
-        args: [-i]  # インプレースで修正
+        args: [-i]