pap 是做什么的？
Interview the human to produce the product layer for a new or existing software project: vision.md, invariants.md (with ADR stubs and protected paths), backlog.md (hypothesis backlog), and a conventions.md skeleton, then recommend the first vertical slice. Use this whenever someone wants to start a new project, product, system, or repo, says "kick off", "set up the product layer", "write the vision", "define invariants", "what should we build first", or invokes /envision, even if they have not m