Stop serializing all extension API calls

[ishaanxgupta]

Problem: xmem-extension/src/api.ts uses one global queue for ingest, search, retrieve, and code queries.

Impact: Read-only autocomplete/retrieval waits behind ingestion, making the extension feel slow.

Fix:

• Queue only mutating graph writes.
• Allow searchMemories, retrieveAnswer, and code queries to run concurrently.
• If needed, use per-user/per-thread keyed write queues.

Acceptance:

• Search/retrieve calls can complete while an ingest is pending.
• No INVALID_CONCURRENT_GRAPH_UPDATE regressions for writes.