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/coding-agent/src/core/sdk.ts

@@ -1,6 +1,6 @@
 import { join } from "node:path";
-import { Agent, type AgentMessage, type ThinkingLevel } from "@earendil-works/pi-agent-core";
-import { clampThinkingLevel, type Message, type Model } from "@earendil-works/pi-ai/compat";
+import { Agent, type AgentMessage, setDefaultStreamFn, type ThinkingLevel } from "@earendil-works/pi-agent-core";
+import { clampThinkingLevel, type Message, type Model, streamSimple } from "@earendil-works/pi-ai/compat";
 import { getAgentDir } from "../config.ts";
 import { resolvePath } from "../utils/paths.ts";
 import { AgentSession } from "./agent-session.ts";
@@ -30,6 +30,11 @@ import {
 	withFileMutationQueue,
 } from "./tools/index.ts";
 
+// Preserve the pre-0.81 fallback for extensions that construct Agent instances
+// or invoke low-level agent loops without supplying streamFn. Agent core remains
+// provider-agnostic and does not import pi-ai/compat itself.
+setDefaultStreamFn(streamSimple);
+
 export interface CreateAgentSessionOptions {
 	/** Working directory for project-local discovery. Default: process.cwd() */
 	cwd?: string;
@@ -294,7 +299,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			tools: [],
 		},
 		convertToLlm: convertToLlmWithBlockImages,
-		streamFunction: async (model, context, options) => {
+		streamFn: async (model, context, options) => {
 			const providerRetrySettings = settingsManager.getProviderRetrySettings();
 			const httpIdleTimeoutMs = settingsManager.getHttpIdleTimeoutMs();
 			// SDKs treat timeout=0 as 0ms (immediate timeout), not "no timeout".