Add design and product documentation

PRODUCT.md

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+# Product — Northline Clinic
+
+Status: approved baseline.
+
+## Audience and job
+
+Patients and caregivers need to understand available planned-care services, prepare safely, choose an appropriate specialist, and request an appointment without medical outcome promises.
+
+## Primary journeys
+
+1. Find the relevant service and understand preparation requirements.
+2. Review specialist credentials and care coordination.
+3. Submit a triage-aware appointment request and receive clear next-step expectations.
+
+## Non-goals
+
+- No diagnosis, emergency triage, guaranteed outcomes, or fabricated patient claims.
+- No collection of unnecessary medical data in public forms.
+
+## Success criteria
+
+Privacy and safety language is visible, booking is understandable, and clinical information remains calm, specific, and accessible.