refactor(config): share agent model schema

src/config/zod-schema.agent-model.ts

@@ -0,0 +1,11 @@
+import { z } from "zod";
+
+export const AgentModelSchema = z.union([
+  z.string(),
+  z
+    .object({
+      primary: z.string().optional(),
+      fallbacks: z.array(z.string()).optional(),
+    })
+    .strict(),
+]);