chore: add project documentation framework

.project-docs/20-architecture/data-flow.md

@@ -0,0 +1,18 @@
+# Data Flow
+
+## Primary Flows
+
+| Flow | Source | Destination | Notes |
+|---|---|---|---|
+
+## State Ownership
+
+- {state owner or persistence rule}
+
+## External Interfaces
+
+- {API, file, service, or user-facing boundary}
+
+## Last Updated
+
+{YYYY-MM-DD}