feat: add docs/ folder (architecture, guides, ADRs)

docs/guides/testing-strategy.md

@@ -0,0 +1,41 @@
+# Testing Strategy
+
+## Test Layers
+
+| Layer | Tool | What to Test |
+|---|---|---|
+| Entities | Vitest (unit) | Zod schema validation, error classes |
+| Use Cases | Vitest (unit) | Business logic with mock implementations via DI |
+| Controllers | Vitest (unit) | Input validation, use case delegation, error mapping |
+| Infrastructure | Vitest (integration) | Real DB via test containers, Payload API calls |
+| UI Components | Vitest + Storybook | Rendering, props, accessibility |
+| Full App | Playwright (E2E) | User flows across both Next.js and TanStack Start |
+
+## Running Tests
+
+```bash
+pnpm test                              # All tests via Turborepo
+cd packages/core && pnpm vitest run    # Core unit tests only
+cd packages/core && pnpm vitest watch  # Core tests in watch mode
+```
+
+## Test Pattern (DI Container)
+
+All tests that use the DI container must initialize and destroy it:
+
+```typescript
+import "reflect-metadata";
+import { beforeEach, afterEach } from "vitest";
+import { initializeContainer, destroyContainer } from "@/di/container.js";
+
+beforeEach(() => { initializeContainer(); });
+afterEach(() => { destroyContainer(); });
+```
+
+This ensures each test gets fresh mock instances (singleton scope resets).
+
+## Test File Location
+
+- Core tests: `packages/core/tests/unit/{use-cases,controllers}/{domain}/`
+- UI tests: co-located next to component (`*.test.tsx`)
+- E2E tests: `tests/e2e/`