Add design and product documentation

PRODUCT.md

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+# Product — SignalDesk AI
+
+Status: approved baseline. Update through a reviewed diff when product intent changes.
+
+## Audience and job
+
+B2B support leaders need to understand incoming demand, automate repetitive resolution work, and prove service quality without hiding decisions behind AI.
+
+## Primary journeys
+
+1. Understand the product and its operational value from the launch page.
+2. Inspect a realistic support dashboard with queue, quality, automation, and escalation evidence.
+3. Request a demo with enough context for a useful sales handoff.
+
+## Non-goals
+
+- No fake live integrations, payments, or unsupported performance claims.
+- No consumer chatbot positioning.
+
+## Success criteria
+
+The value proposition is clear above the fold, dashboard examples remain credible, and every primary page leads naturally to a demo decision.