feat(coding-agent): replace model registry with model runtime

Move provider auth and OAuth flows onto pi-ai Models, compose models.json and extension overlays through ModelRuntime, and retain ModelRegistry as an extension compatibility facade.

packages/ai/test/oauth-device-code.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
-import { pollOAuthDeviceCodeFlow } from "../src/utils/oauth/device-code.ts";
+import { pollOAuthDeviceCodeFlow } from "../src/auth/oauth/device-code.ts";
 
 describe("OAuth device-code polling", () => {
 	afterEach(() => {