feat(gateway): expose frontend memory control plane

[x-lixu]

Summary

• add a provider-neutral, owner-scoped GET/PATCH memory control plane over the existing FrontendMemoryRuntime
• let the smart-cockpit client list and exactly delete the same USER/MEMORY entries used by Realtime
• retry automatic memory extraction once without optional temperature when an OpenAI-compatible model rejects it with HTTP 400
• publish the additive 5.6.0 health contract and document the client/provider boundaries

Validation

• npm run lint
• npm test
• npm run build
• npm run example:smart-cockpit:build
• targeted Gateway API and memory extractor tests