fix(agent): restore streamFn extension compatibility

Keep streamFn required for typed callers while preserving the legacy runtime fallback for extensions that omit it.\n\nfixes #6915

packages/agent/src/stream-fn.ts

@@ -0,0 +1,20 @@
+import type { StreamFn } from "./types.ts";
+
+let defaultStreamFn: StreamFn | undefined;
+
+/**
+ * Configure the fallback used by Agent and low-level loops when callers omit streamFn.
+ *
+ * Hosts that provide a default model runtime can install its stream function here
+ * without making pi-agent-core depend on a provider catalog or compatibility layer.
+ */
+export function setDefaultStreamFn(streamFn: StreamFn | undefined): void {
+	defaultStreamFn = streamFn;
+}
+
+export function getDefaultStreamFn(): StreamFn {
+	if (!defaultStreamFn) {
+		throw new Error("No default stream function configured. Pass streamFn explicitly or call setDefaultStreamFn().");
+	}
+	return defaultStreamFn;
+}