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

pnpm-lock.yaml

@@ -760,9 +760,6 @@ importers:
 
   packages/core-trpc:
     dependencies:
-      "@repo/core-api":
-        specifier: workspace:*
-        version: link:../core-api
       "@tanstack/react-query":
         specifier: ^5.66.0
         version: 5.96.2(react@19.2.4)