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/render-with-providers.tsx

@@ -7,6 +7,12 @@ export interface RenderOptions {
   queryClient?: QueryClient;
 }
 
+// Wraps the given UI with QueryClientProvider only.
+// This helper intentionally omits a TRPCProvider. Adding one would require importing
+// AppRouter from @repo/core-api, which violates the tooling→core-composition boundary rule.
+// For components that need tRPC in the render tree, the consumer must:
+//   1. Wire their own TRPCProvider (from their app's tRPC client setup).
+//   2. Pass a client built with createMockTrpcClient as the tRPC client.
 export function renderWithProviders(
   ui: ReactElement,
   options: RenderOptions = {},