fix: align memory vector DB typing with node sqlite

Regeneration-Prompt: |
  Mainline check/prep was failing in memory-core because replaceMemoryVectorRow
  accepted a handwritten database shape whose prepare().run signature used
  unknown[] and was not assignable from node:sqlite DatabaseSync. Keep the
  helper behavior the same, preserve the existing call sites and test, and fix
  the type seam by aligning the helper with the real node:sqlite prepare type
  instead of widening individual callers.

extensions/memory-core/src/memory/manager-vector-write.ts

@@ -1,8 +1,6 @@
-type VectorWriteDb = {
-  prepare: (sql: string) => {
-    run: (...params: unknown[]) => void;
-  };
-};
+import type { DatabaseSync } from "node:sqlite";
+
+type VectorWriteDb = Pick<DatabaseSync, "prepare">;
 
 const vectorToBlob = (embedding: number[]): Buffer =>
   Buffer.from(new Float32Array(embedding).buffer);