feat(sdk): stage 5 — export adapter factories from package root

Expose the concrete adapter factories so consumers no longer reach into deep
adapter paths:
- createHeadlessAdapter — no-op PreviewAdapter for agents/CI/SSR (no browser)
- createMemoryAdapter — in-memory PersistAdapter for tests/headless open
- createFsAdapter (+ FsAdapterOptions) — node fs PersistAdapter for local dev

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: vanceingalls

packages/sdk/src/index.ts

@@ -32,3 +32,9 @@ export { createPersistQueue } from "./persist-queue.js";
 export type { PersistQueueModule, PersistQueueOptions } from "./persist-queue.js";
 
 export type { PersistAdapter, PreviewAdapter, PersistVersionEntry } from "./adapters/types.js";
+
+// Concrete adapter factories.
+export { createMemoryAdapter } from "./adapters/memory.js";
+export { createHeadlessAdapter } from "./adapters/headless.js";
+export { createFsAdapter } from "./adapters/fs.js";
+export type { FsAdapterOptions } from "./adapters/fs.js";