arc-agent: AI System Design Generator

Finally a structured approach to system design: arc-agent. It is a Go CLI for turning a system design prompt into a real design workspace: requirements, entities, APIs, high-level design, optional deep dives, and architecture diagrams. The core idea is simple: do not ask an agent for one big architecture document and hope it is coherent. Split the work into stages, validate every artifact, repair failures, and render the result into files that are easy to review.