docs: trpc now optional — prerequisite notes + conditional HTML

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-ui/AGENTS.md

@@ -20,6 +20,8 @@ Feature-specific organisms (e.g., `ArticleCard`, `ArticleList`, `HeaderNavMenu`)
 - Any app package
 - `@repo/core-api`, `@repo/core-cms`, `@repo/core-trpc`
 
+> Note: `@repo/core-trpc` is an optional package scaffolded via `pnpm turbo gen core-package trpc`. If not present, this constraint still applies to any future installation.
+
 ## Public exports
 
 From `package.json`: