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/src/oauth.ts

@@ -1 +1,10 @@
-export * from "./utils/oauth/index.ts";
+/** Type-only compatibility entry point for coding-agent extension OAuth declarations. */
+export type {
+	OAuthAuthInfo,
+	OAuthCredentials,
+	OAuthDeviceCodeInfo,
+	OAuthLoginCallbacks,
+	OAuthPrompt,
+	OAuthSelectOption,
+	OAuthSelectPrompt,
+} from "./compat/extension-oauth-types.ts";