refactor(core-trpc): make the tRPC context router-agnostic

Ports the type-safety/ergonomics half of the upstream cycle-break: core-trpc
no longer imports AppRouter from @repo/core-api (an illegal
core -> core-composition boundary edge). The runtime context and both
providers are now AnyTRPCRouter-generic; consumers inject their router type
via useTRPC<TRouter>().

The clean-slate tree is already cycle-free (no kept feature consumes
core-trpc), so this is purely the architecture/boundary improvement. The
per-feature app-slice types the upstream commit added for the demo features
are intentionally skipped: no kept feature consumes useTRPC yet.

Also covers the router-agnostic useTRPC delegation: the refactor turned
useTRPC into a wrapper function whose body was unexercised, dropping
client.ts below the coverage floor — client.test.ts stubs the context
factory and asserts the delegation, restoring client.ts to 100%.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

packages/core-trpc/src/client.test.ts

@@ -1,4 +1,15 @@
-import { describe, it, expect } from "vitest";
+import { describe, it, expect, vi } from "vitest";
+
+// Stub the tRPC context factory so useTRPC() can run outside a React render:
+// the real context.useTRPC() requires a mounted TRPCProvider. The stubbed
+// proxy lets us exercise the router-agnostic delegation (client.ts:useTRPC).
+vi.mock("@trpc/tanstack-react-query", () => ({
+  createTRPCContext: () => ({
+    TRPCProvider: () => null,
+    useTRPC: () => ({ __proxy: "trpc-options-proxy" }),
+  }),
+}));
+
 import { useTRPC, TRPCProvider } from "./client";
 
 describe("core-trpc client exports", () => {
@@ -9,4 +20,10 @@ describe("core-trpc client exports", () => {
   it("exports TRPCProvider component", () => {
     expect(TRPCProvider).toBeTypeOf("function");
   });
+
+  it("useTRPC delegates to the underlying router-agnostic tRPC proxy", () => {
+    // The generic is a compile-time-only cast; at runtime it returns whatever
+    // the shared context proxy yields.
+    expect(useTRPC()).toEqual({ __proxy: "trpc-options-proxy" });
+  });
 });