Add design and product documentation

PRODUCT.md

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+# Product — Monochrome Supply
+
+Status: approved baseline.
+
+## Audience and job
+
+Style-conscious customers need to discover a focused collection, inspect garment detail and fit, choose a valid variant, and understand delivery and returns before purchase.
+
+## Primary journeys
+
+1. Browse editorial collection stories and product categories.
+2. Evaluate product imagery, material, fit, size, color, price, and availability.
+3. Add a valid variant to cart and review shipping and return expectations.
+
+## Non-goals
+
+- No fake scarcity, unavailable checkout integrations, or decorative controls without state.
+
+## Success criteria
+
+Products dominate the interface, variant state is unambiguous, and mobile shopping remains fast and readable.