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/package.json

@@ -23,7 +23,6 @@
     "@testing-library/react": "^16.0.0",
     "@testing-library/user-event": "^14.5.0",
     "@trpc/client": "^11.0.0",
-    "@trpc/tanstack-react-query": "^11.0.0",
     "@tanstack/react-query": "^5.59.0",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",