fix: gate xhigh by model (#444) (thanks @grp06)

src/agents/model-selection.ts

@@ -7,7 +7,13 @@ export type ModelRef = {
   model: string;
 };
 
-export type ThinkLevel = "off" | "minimal" | "low" | "medium" | "high";
+export type ThinkLevel =
+  | "off"
+  | "minimal"
+  | "low"
+  | "medium"
+  | "high"
+  | "xhigh";
 
 export type ModelAliasIndex = {
   byAlias: Map<string, { alias: string; ref: ModelRef }>;