Prove or disprove an idea by building the smallest throwaway artifact that could falsify it, in an isolated worktree, ending in a PROVEN / DISPROVEN / CONDITIONAL verdict backed by evidence - never a PR. Use when the user wants to "spike this", "build a proof of concept", "POC this", "prototype it to find out", asks "is this possible", "is this feasible", "does this fit our stack", or "would it be worth changing X to allow Y" and wants it settled by building rather than by analysis, wants to compare approaches on evidence before picking one, or invokes $prp-spike.