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/eslint.config.js

@@ -0,0 +1,11 @@
+import baseConfig from "@repo/core-eslint/base";
+
+export default [
+  ...baseConfig,
+  {
+    rules: {
+      // test utilities are intended to be used in test contexts
+      "no-console": "off",
+    },
+  },
+];