prefetch() now reuses the in-flight background search when it matches
the current query, or falls back to a synchronous search (capped at 3s,
split across both requests in case-insensitive mode) so memories are
injected on the same turn instead of one turn behind.
- Track _prefetch_query to scope cached results to their query
- Reset _prefetch_result when starting a new background search to
prevent stale cross-turn injection after a timed-out join
- Guard background thread writes against superseded queries
- Add optional per-request timeout override to LocalMem0Client
- Update AGENTS.md and README to describe the same-turn flow
ARIA