claudetool: add simplified patch support

For weaker models.

Also, improve fallback parsing introduced earlier.
diff --git a/llm/llm.go b/llm/llm.go
index ffaad3e..19a1d8d 100644
--- a/llm/llm.go
+++ b/llm/llm.go
@@ -21,6 +21,18 @@
 	TokenContextWindow() int
 }
 
+type SimplifiedPatcher interface {
+	// UseSimplifiedPatch reports whether the service should use the simplified patch input schema.
+	UseSimplifiedPatch() bool
+}
+
+func UseSimplifiedPatch(svc Service) bool {
+	if sp, ok := svc.(SimplifiedPatcher); ok {
+		return sp.UseSimplifiedPatch()
+	}
+	return false
+}
+
 // MustSchema validates that schema is a valid JSON schema and returns it as a json.RawMessage.
 // It panics if the schema is invalid.
 // The schema must have at least type="object" and a properties key.