graph-service: agent memory ontology + reliable async ingest

[yulongbai-nov]

Summary\n- Adds an agent-memory ontology registry (agent_memory_v1) and optional schema_id on POST /messages.\n- Auto-selects agent_memory_v1 when message content contains <graphiti_episode.\n- Hardens async ingestion by keeping a single Graphiti instance alive for background jobs and running the worker at app lifespan.\n\n## Testing\n- make check (core)\n- make check (server)\n\n## Docs\n- Updated server/README.md and added examples/agent_memory_ontology/README.md.\n\n## Specs\n- .specs/agent-memory-ontology/

[danielchalef]

All contributors have signed the CLA ✍️ ✅
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[yulongbai-nov]

I have read the CLA Document and I hereby sign the CLA