test_build: init the fixture repo through the git helper

.claude/test/test_build.py

diff --git a/.claude/test/test_build.py b/.claude/test/test_build.py
index 75c88fe..826bf78 100644
--- a/.claude/test/test_build.py
+++ b/.claude/test/test_build.py
@@ -741,8 +741,8 @@
     @classmethod
     def repo(cls, path):
         """A fresh repository with one commit; its HEAD."""
-        import subprocess
-        subprocess.run(['git', 'init', '-q', str(path)], check=True, env=cls.CLEAN_ENV)
+        path.mkdir()
+        cls.git(path, 'init', '-q')
         cls.git(path, 'commit', '-q', '--allow-empty', '-m', str(path))
         return cls.git(path, 'rev-parse', 'HEAD')