feat(ai): add OpenRouter provider routing support

Allows custom models to specify which upstream providers OpenRouter
should route requests to via the `openRouterRouting` field in model
definitions.

Supported fields:
- `only`: list of provider slugs to exclusively use
- `order`: list of provider slugs to try in order

packages/coding-agent/src/core/model-registry.ts

@@ -25,6 +25,12 @@ import type { AuthStorage } from "./auth-storage.js";
 
 const Ajv = (AjvModule as any).default || AjvModule;
 
+// Schema for OpenRouter routing preferences
+const OpenRouterRoutingSchema = Type.Object({
+	only: Type.Optional(Type.Array(Type.String())),
+	order: Type.Optional(Type.Array(Type.String())),
+});
+
 // Schema for OpenAI compatibility settings
 const OpenAICompletionsCompatSchema = Type.Object({
 	supportsStore: Type.Optional(Type.Boolean()),
@@ -37,6 +43,7 @@ const OpenAICompletionsCompatSchema = Type.Object({
 	requiresThinkingAsText: Type.Optional(Type.Boolean()),
 	requiresMistralToolIds: Type.Optional(Type.Boolean()),
 	thinkingFormat: Type.Optional(Type.Union([Type.Literal("openai"), Type.Literal("zai")])),
+	openRouterRouting: Type.Optional(OpenRouterRoutingSchema),
 });
 
 const OpenAIResponsesCompatSchema = Type.Object({