feat(core-testing): scaffold shared testing utilities package

Adds @repo/core-testing (tag: tooling) with:
- factory/defineFactory: monotonic-sequence object factories with overrides
- contract/defineContractSuite: shared test suites runnable against multiple impls
- react/renderWithProviders + createMockTrpcClient: RTL helpers
- payload/stubPayloadConfig + mockPayloadModule: Payload mocking helpers
- setup/{jsdom,node}: vitest setup files

Spec: docs/superpowers/specs/2026-05-05-tdd-foundation-design.md §5

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-testing/AGENTS.md

@@ -0,0 +1,28 @@
+# @repo/core-testing
+
+Shared testing utilities. Tag: `tooling`. May be depended on by any package as a devDependency.
+
+## Subpath exports
+
+- `@repo/core-testing/factory` — `defineFactory<T>(builder)` for test data factories
+- `@repo/core-testing/contract` — `defineContractSuite<T>(name, suite)` for cross-impl contract tests
+- `@repo/core-testing/react` — `renderWithProviders`, `createMockTrpcClient`
+- `@repo/core-testing/payload` — `stubPayloadConfig`, `mockPayloadModule`
+- `@repo/core-testing/setup/jsdom` — vitest setupFile (jest-dom + cleanup)
+- `@repo/core-testing/setup/node` — vitest setupFile (no-op placeholder)
+
+## Adding a factory
+
+```typescript
+import { defineFactory } from "@repo/core-testing/factory";
+
+export const articleFactory = defineFactory<Article>(({ sequence }) => ({
+  id: `article-${sequence}`,
+  title: `Article ${sequence}`,
+  // stable defaults — overrides drive variation
+}));
+```
+
+## Adding a contract suite
+
+See `docs/guides/tdd-workflow.md` §"Contract suite usage".