docs(adr): add 0006-demo-user-three-layer-policy

docs/adr/0006-demo-user-three-layer-policy.md

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+# ADR-0006: Demo-user write protection enforced in three layers
+
+## Status
+
+accepted
+
+## Context
+
+Scrum4Me has a demo account that allows prospective users to explore the app without signing up. The demo user must never be able to create, update, or delete any data. A single guard at one layer is insufficient: a bug or a missing check in any one layer would expose a write path. See `docs/architecture/auth-and-sessions.md` and `docs/plans/ST-1110-demo-readonly.md` for implementation details.
+
+## Decision
+
+Write protection for the demo user is enforced at **three independent layers**:
+
+1. **Network — `proxy.ts`:** The Next.js proxy middleware rejects all non-GET requests from demo sessions before they reach any route handler or server action.
+2. **Server — every Server Action and Route Handler:** Each write endpoint checks `session.isDemo` and returns `403` immediately if true.
+3. **UI — disabled buttons + `<DemoTooltip>`:** Write controls (create, edit, delete, reorder) are rendered as `disabled` with a tooltip explaining the demo restriction. No write request is ever sent.
+
+## Consequences
+
+### Positive
+
+- Defense-in-depth: any single layer can fail independently without exposing a write path.
+- Clear user feedback at the UI layer without relying on error responses.
+- Straightforward to audit: search for `isDemo` to find all enforcement points.
+
+### Negative
+
+- Three enforcement sites for every new write operation — easy to miss one when adding a new feature.
+- Mitigation: the `DemoTooltip` pattern is documented in `docs/patterns/` and enforced in code review.