refactor: make memory embedding adapters generic

src/plugin-sdk/memory-core-engine-runtime.ts

@@ -0,0 +1,7 @@
+// Thin engine runtime compat surface for the bundled memory-core plugin.
+// Keep extension-owned engine exports isolated behind a dedicated SDK subpath.
+
+export {
+  getMemorySearchManager,
+  MemoryIndexManager,
+} from "../../extensions/memory-core/src/memory/index.js";