claudetool: remove knowledge base, focus on about sketch

We should have a more general kb.
Meanwhile, this is important and standalone.
Make it all clearer and sharper.
diff --git a/loop/agent.go b/loop/agent.go
index 8992bb3..c83dad1 100644
--- a/loop/agent.go
+++ b/loop/agent.go
@@ -956,7 +956,7 @@
 	}
 
 	if experiment.Enabled("kb") {
-		convo.Tools = append(convo.Tools, claudetool.KnowledgeBase)
+		convo.Tools = append(convo.Tools, claudetool.AboutSketch)
 	}
 
 	// One-shot mode is non-interactive, multiple choice requires human response