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/providers/cloudflare-stream.ts

@@ -0,0 +1,28 @@
+import type { Api, Model, ProviderEnv, ProviderStreams } from "../types.ts";
+
+const CLOUDFLARE_ACCOUNT_ID = "CLOUDFLARE_ACCOUNT_ID";
+const CLOUDFLARE_GATEWAY_ID = "CLOUDFLARE_GATEWAY_ID";
+
+export function resolveCloudflareModel<TApi extends Api>(
+	model: Model<TApi>,
+	env: ProviderEnv | undefined,
+): Model<TApi> {
+	if (!env) return model;
+	const baseUrl = model.baseUrl
+		.replaceAll(`{${CLOUDFLARE_ACCOUNT_ID}}`, env[CLOUDFLARE_ACCOUNT_ID] ?? `{${CLOUDFLARE_ACCOUNT_ID}}`)
+		.replaceAll(`{${CLOUDFLARE_GATEWAY_ID}}`, env[CLOUDFLARE_GATEWAY_ID] ?? `{${CLOUDFLARE_GATEWAY_ID}}`);
+	return baseUrl === model.baseUrl ? model : { ...model, baseUrl };
+}
+
+/**
+ * Wrap an API implementation so Cloudflare account/gateway endpoint
+ * placeholders materialize from the resolved provider env before dispatch.
+ */
+export function cloudflareStreams(streams: ProviderStreams): ProviderStreams {
+	return {
+		stream: (model, context, options) =>
+			streams.stream(resolveCloudflareModel(model, options?.env), context, options),
+		streamSimple: (model, context, options) =>
+			streams.streamSimple(resolveCloudflareModel(model, options?.env), context, options),
+	};
+}