Isolate historical rejection fixture from the installed fleet hook

tests/test_generated_commit_guard.py

@@ -31,6 +31,11 @@ def test_ci_checks_each_commit_in_range(tmp_path):
     def git(*args):
         return subprocess.check_output(["git", *args], cwd=tmp_path).decode().strip()
     git("init", "-q")
+    # This fixture deliberately creates a rejected historical commit so CI's
+    # range checker can inspect it. Isolate it from the workstation's real hook.
+    hooks = tmp_path / "fixture-hooks"
+    hooks.mkdir()
+    git("config", "core.hooksPath", str(hooks))
     git("config", "user.name", "Test")
     git("config", "user.email", "test@example.test")
     (tmp_path / "source.py").write_text("source\n")