Server-side duplicate detection on store_memory

Saving a memory that exactly matches one already stored in the same space now returns the existing memory instead of creating a copy, so a retried save from any connected client (ChatGPT, Claude, Cursor, Codex) is always safe. When a new memory is very similar to — but not identical to — an existing one, the response now includes a quiet note pointing at the similar memory so you can clean it up if it was unintentional.