Agent Skills: ring:writing-prds

Writing a Product Requirements Document that explains to the squad WHAT is being built and WHY: problem, explicit scope in/out, functional requirements, and testable acceptance criteria. Gate 1 of ring:using-pm-team; runs after ring:researching-features and stays technology-free (no architecture, frameworks, or schemas). Use when starting a new feature or asked to plan or produce requirements. Skip when a validated PRD exists, for pure technical changes, or bug fixes.