Produce a concise architecture and onboarding overview of an unfamiliar codebase. Use when the user asks to "explain this repo", "give me an overview", "how is this project structured", "where do I start", "onboard me to this codebase", "what does this project do", or opens a project they don't know. Maps the stack, entry points, key modules, data flow, and the first files a new contributor should read.
