Review follow-up: migrate LocalVectorStore to async fs I/O

[amondnet]

Original Review Suggestion

Using writeFileSync (and readFileSync) blocks the event loop. Since these are called within async methods, it is recommended to use the asynchronous node:fs/promises API to avoid blocking the application, especially as the vector index grows.

Reviewer: @gemini-code-assist
PR: #92
File: packages/store/src/local/vector-store.ts (line 124)
Comment: #92 (comment)

Status

Decision: Skipped
Reason: This store is a zero-dependency dev-fallback for small JSON files where sync I/O is acceptable. Migrating to node:fs/promises would be a significant refactor with no practical benefit at current scale.

Action Items

• Migrate readFileSync / writeFileSync in open() / flush() to node:fs/promises equivalents
• Update flush() to return Promise<void> and make callers await it
• Revisit if LocalVectorStore starts being used with large indexes in production paths