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>

pnpm-lock.yaml

@@ -443,9 +443,6 @@ importers:
       '@trpc/server':
         specifier: ^11.0.0
         version: 11.16.0(typescript@5.9.3)
-      '@trpc/tanstack-react-query':
-        specifier: ^11.0.0
-        version: 11.16.0(@tanstack/react-query@5.96.2(react@19.2.4))(@trpc/client@11.16.0(@trpc/server@11.16.0(typescript@5.9.3))(typescript@5.9.3))(@trpc/server@11.16.0(typescript@5.9.3))(react@19.2.4)(typescript@5.9.3)
       payload:
         specifier: ^3.0.0
         version: 3.81.0(graphql@16.13.2)(typescript@5.9.3)