claudetool: don't attempt to install zero tools

Follow-up to 495c1fa247565e21b36bcb847c6cd3f08e0e196f

Fixes #127
diff --git a/claudetool/bash.go b/claudetool/bash.go
index 39914c6..ce27ec7 100644
--- a/claudetool/bash.go
+++ b/claudetool/bash.go
@@ -353,6 +353,10 @@
 		missing = append(missing, cmd)
 	}
 
+	if len(missing) == 0 {
+		return nil
+	}
+
 	err = b.installTools(ctx, missing)
 	if err != nil {
 		return err