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/src/react/mock-trpc.ts

@@ -18,6 +18,9 @@ export function createMockTrpcClient<TRouter extends AnyTRPCRouter>(
     return new Response(JSON.stringify([{ result: { data: superjson.serialize(result) } }]), { status: 200 });
   };
 
+  // httpBatchLink<TRouter> requires a conditional TransformerOptions<TRouter> that
+  // depends on whether the router uses a transformer; making this generic-friendly
+  // without parameterizing twice requires <any> here.
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   const link = httpBatchLink<any>({
     url: "http://mock/api/trpc",