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/loop/agent.go b/loop/agent.go
index 1ce67e5..2f4efe9 100644
--- a/loop/agent.go
+++ b/loop/agent.go
@@ -598,7 +598,10 @@
 <git_root>
 %v
 </git_root>
-`, editPrompt, a.config.ClientGOOS, a.config.ClientGOARCH, a.workingDir, a.repoRoot)
+<HEAD>
+%v
+</HEAD>
+`, editPrompt, a.config.ClientGOOS, a.config.ClientGOARCH, a.workingDir, a.repoRoot, a.initialCommit)
 
 	// Register all tools with the conversation
 	// When adding, removing, or modifying tools here, double-check that the termui tool display