claudetool: add "related files" detection to codereview tool

diff --git a/claudetool/testdata/new_test_build_error.txtar b/claudetool/testdata/new_test_build_error.txtar
index 2fe1d34..2405c49 100644
--- a/claudetool/testdata/new_test_build_error.txtar
+++ b/claudetool/testdata/new_test_build_error.txtar
@@ -19,6 +19,8 @@
 Add test with build error
 
 -- .run_test --
+# Errors
+
 Test regressions detected between initial commit (cf75ed70e940ce33a17a70b1894ad053b67731c0) and HEAD:
 
 1: sketch.dev: Previously had no tests, now has build/vet errors