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-eslint/base.js

@@ -22,6 +22,7 @@ export default [
         { type: "app", pattern: "apps/*" },
         { type: "tooling", pattern: "packages/core-eslint" },
         { type: "tooling", pattern: "packages/core-typescript" },
+        { type: "tooling", pattern: "packages/core-testing" },
         { type: "core-composition", pattern: "packages/core-api" },
         { type: "core-composition", pattern: "packages/core-cms" },
         { type: "core", pattern: "packages/core-*" },