feat(extensions): support async extension factory functions

Extensions can now use async initialization, enabling:
- Dynamic imports (e.g., loading tools from external packages)
- Async setup (config fetching, service connections)
- Lazy-loaded dependencies

Changes:
- ExtensionFactory type now returns void | Promise<void>
- loadExtensionFromFactory is now async, returns Promise<LoadedExtension>
- All factory(api) calls are now awaited

Backwards compatible: sync extensions continue to work unchanged.

packages/coding-agent/src/core/extensions/loader.ts

@@ -313,7 +313,7 @@ async function loadExtensionWithBun(
 			setFlagValue,
 		} = createExtensionAPI(handlers, tools, cwd, extensionPath, eventBus, sharedUI);
 
-		factory(api);
+		await factory(api);
 
 		return {
 			extension: {
@@ -401,7 +401,7 @@ async function loadExtension(
 			setFlagValue,
 		} = createExtensionAPI(handlers, tools, cwd, extensionPath, eventBus, sharedUI);
 
-		factory(api);
+		await factory(api);
 
 		return {
 			extension: {
@@ -436,13 +436,13 @@ async function loadExtension(
 /**
  * Create a LoadedExtension from an inline factory function.
  */
-export function loadExtensionFromFactory(
+export async function loadExtensionFromFactory(
 	factory: ExtensionFactory,
 	cwd: string,
 	eventBus: EventBus,
 	sharedUI: { ui: ExtensionUIContext; hasUI: boolean },
 	name = "<inline>",
-): LoadedExtension {
+): Promise<LoadedExtension> {
 	const handlers = new Map<string, HandlerFn[]>();
 	const tools = new Map<string, RegisteredTool>();
 	const {
@@ -464,7 +464,7 @@ export function loadExtensionFromFactory(
 		setFlagValue,
 	} = createExtensionAPI(handlers, tools, cwd, name, eventBus, sharedUI);
 
-	factory(api);
+	await factory(api);
 
 	return {
 		path: name,

packages/coding-agent/src/core/extensions/types.ts

@@ -641,8 +641,8 @@ export interface ExtensionAPI {
 	events: EventBus;
 }
 
-/** Extension factory function type. */
-export type ExtensionFactory = (pi: ExtensionAPI) => void;
+/** Extension factory function type. Supports both sync and async initialization. */
+export type ExtensionFactory = (pi: ExtensionAPI) => void | Promise<void>;
 
 // ============================================================================
 // Loaded Extension Types

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

@@ -488,7 +488,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		};
 		for (let i = 0; i < options.extensions.length; i++) {
 			const factory = options.extensions[i];
-			const loaded = loadExtensionFromFactory(factory, cwd, eventBus, uiHolder, `<inline-${i}>`);
+			const loaded = await loadExtensionFromFactory(factory, cwd, eventBus, uiHolder, `<inline-${i}>`);
 			extensionsResult.extensions.push(loaded);
 		}
 		// Extend setUIContext to update inline extensions too