claudetool: add "related files" detection to codereview tool

diff --git a/claudetool/testdata/gopls_issues.txtar b/claudetool/testdata/gopls_issues.txtar
index 56f1557..c0250ac 100644
--- a/claudetool/testdata/gopls_issues.txtar
+++ b/claudetool/testdata/gopls_issues.txtar
@@ -15,6 +15,8 @@
 Add file with gopls issues
 
 -- .run_test --
+# Errors
+
 Gopls check issues detected:
 
 1. /PATH/TO/REPO/p.go:5:2-22: unreachable code