Use when a product feature or initiative needs a written requirements doc before building — capturing problem, users, scope, non-goals, and success metrics so engineering and stakeholders align. Triggers: "write a PRD", "product requirements", "spec out this feature", "what are we building and why", kickoff of a product feature, or turning a brainstorming session into a durable artifact. Produces a PRD and offers to file it in Linear/Notion. For the *technical* design that follows, use `tech-spec`; to split the work, use `vertical-slicing`.
