Add design and product documentation

PRODUCT.md

@@ -0,0 +1,13 @@
+# Product
+
+Status: draft required before major implementation.
+
+## Required decisions
+
+- Audience and primary job-to-be-done.
+- Three most important user journeys.
+- Required pages, content, data, and integrations.
+- Explicit non-goals and legal or operational constraints.
+- Measurable acceptance criteria.
+
+The AI must draft this document from supplied specs and conversation, show the diff, and receive explicit approval before treating it as authoritative.