Fix vector persistence and tighten platform docs

[kevintseng]

Summary

• Fix sqlite-vec vector persistence by binding vec0 rowids as BigInt and replacing vectors via delete+insert.
• Prevent archived entities from being re-indexed by late background embeddings and apply archive/tag/namespace filters to vector recall hydration.
• Flush queued embeddings for short-lived CLI remember before closing the database.
• Tighten README/platform docs around the actual local MeMesh plugin surface.

Verification

• npm run typecheck
• npm test (29 files / 449 tests)
• npm run build
• npm run test:packaged
• CLI duplicate remember smoke: same entity remained one entities_vec row with no vector warnings.