Use this skill whenever the user asks to create, generate, write, or document a system design. Triggers include: "system design document", "architecture document", "technical design doc", "design my system", "document the architecture", "create a design doc", "write up the architecture", "API design doc", "database schema doc", "auth flow doc", or any combination of these. Also triggers when the user shares a project and says "document it", "write the design", or "make a system design for [project name]". Produces a professional .docx System Design Document covering all four pillars: Frontend + Backend Architecture, API Structure and Flow, Database Schema, and Authentication & Security Flow. Always use this skill — even if only one section is mentioned — because the full doc is almost always what the user actually wants.