Add design and product documentation

PRODUCT.md

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+# Product — Common Ground
+
+Status: approved baseline.
+
+## Audience and job
+
+Supporters, volunteers, partners, and journalists need to verify where resources go, understand active campaigns, and take a concrete next action.
+
+## Primary journeys
+
+1. Discover a campaign through evidence-backed reporting.
+2. Inspect budgets, progress, documents, and human stories.
+3. Donate, volunteer, or share with informed consent.
+
+## Non-goals
+
+- No vague impact totals without methodology or source documents.
+- No exploitative imagery or manipulative urgency.
+
+## Success criteria
+
+Every campaign connects narrative to verifiable evidence, and action paths explain commitment, privacy, and follow-up.