claudetool: improve codereview
Do a bunch of un-vibecoding and bug fixing.
Unfortunately, lots left.
Get rid of vet; gopls check covers it.
Add testing infrastructure and a bunch of fixtures.
diff --git a/claudetool/testdata/multiple_format_issues.txtar b/claudetool/testdata/multiple_format_issues.txtar
new file mode 100644
index 0000000..0024a48
--- /dev/null
+++ b/claudetool/testdata/multiple_format_issues.txtar
@@ -0,0 +1,21 @@
+Test that formatting issues are not reported if pre-existing, per-file
+
+-- main.go --
+ package main
+
+-- .commit --
+Initial commit
+
+-- main.go --
+ package main
+
+ import "fmt"
+
+-- file1.go --
+ package main
+
+-- .commit --
+Add multiple files with formatting issues
+
+-- .run_autoformat --
+/PATH/TO/REPO/file1.go