llm/ant: add Claude 4 sonnet/opus

Not set as default yet.
There's a bit of prompt engineering work
to do before we flip the switch.
diff --git a/llm/llm.go b/llm/llm.go
index 3de6b7f..0e14c7f 100644
--- a/llm/llm.go
+++ b/llm/llm.go
@@ -169,6 +169,7 @@
 	StopReasonMaxTokens
 	StopReasonEndTurn
 	StopReasonToolUse
+	StopReasonRefusal
 )
 
 type Response struct {