Build a non-ambiguous, complete, provenance-tagged product spec for a feature area — the behavioral oracle that prove-behavior tests against. Researches every reachable source (code, live product, sibling specs, Jira, Confluence, Slack, git history) to build a mental map, then interrogates the user exhaustively to resolve EVERY ambiguity before writing. Use when: (1) user says 'feature-spec', 'write the spec for X', 'build the product spec', 'spec out this feature', (2) before running prove-behavior on a feature that has no spec yet, (3) onboarding onto a feature and wanting the ground-truth contract. One spec per feature AREA (e.g. all of the settings screen, or checkout, or the text editor). Output: docs/features/<area>/product-spec.md.