Produces a comprehensive system design document — a buildable technical blueprint covering requirements, architecture, data model, API, data flows, failure modes, operational and security concerns, and open questions. Use when the user says "write a system design", "design doc", "technical design for [system]", or describes a new service they need to document before coding. Use this for the full build-from blueprint of one system; use api-contract-design for the API surface in isolation, data-model-design for the schema in isolation, and technical-vision-document for multi-system 12-24 month direction rather than one implementation.
