Add extensions option to createAgentSession SDK

- Accept ExtensionFactory[] for inline extensions (merged with discovery)
- Mark preloadedExtensions as @internal (CLI implementation detail)
- Update sdk.md with inline extension example
- Update CHANGELOG

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

@@ -2,7 +2,7 @@
  * Extension system for lifecycle events and custom tools.
  */
 
-export { discoverAndLoadExtensions, loadExtensions } from "./loader.js";
+export { discoverAndLoadExtensions, loadExtensionFromFactory, loadExtensions } from "./loader.js";
 export type { BranchHandler, ExtensionErrorListener, NavigateTreeHandler, NewSessionHandler } from "./runner.js";
 export { ExtensionRunner } from "./runner.js";
 export type {