claudetool: add "related files" detection to codereview tool

diff --git a/claudetool/testdata/gopls_new_issue_with_existing.txtar b/claudetool/testdata/gopls_new_issue_with_existing.txtar
index d47fb02..594ea93 100644
--- a/claudetool/testdata/gopls_new_issue_with_existing.txtar
+++ b/claudetool/testdata/gopls_new_issue_with_existing.txtar
@@ -26,6 +26,8 @@
 Add a new function with a new gopls issue
 
 -- .run_test --
+# Errors
+
 Gopls check issues detected:
 
 1. /PATH/TO/REPO/p.go:9:2-31: unreachable code