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/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