docs: trpc now optional — prerequisite notes + conditional HTML

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

packages/core-cms/AGENTS.md

@@ -24,6 +24,8 @@
 - Any app package
 - `@repo/core-shared`, `@repo/core-api`, `@repo/core-trpc`, `@repo/core-ui`
 
+> Note: `@repo/core-trpc` and `@repo/core-ui` are optional packages scaffolded via `pnpm turbo gen core-package trpc` / `ui`. If not present, these constraints still apply to any future installation.
+
 ## Public exports
 
 From `package.json`: