fix(core-testing): address code review feedback for Task 1

- Remove unused @trpc/tanstack-react-query dependency
- Document renderWithProviders tRPC provider omission (boundary constraint)
- Implement deep merge in defineFactory (preserves nested sibling keys)
- Document httpBatchLink<any> rationale in mock-trpc.ts
- Align core-testing's own vitest.config with safety defaults (mockReset, unstubGlobals)
- Add createMockTrpcClient usage example to AGENTS.md

Reviewer: superpowers:code-reviewer (Task 1 of Plan 7).

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

packages/core-testing/vitest.config.ts

@@ -9,6 +9,8 @@ export default defineConfig({
     setupFiles: ["./src/setup/jsdom.ts"],
     clearMocks: true,
     restoreMocks: true,
+    mockReset: true,
+    unstubGlobals: true,
   },
   resolve: {
     alias: { "@": path.resolve(__dirname, "./src") },