docs: ui now optional — prerequisite notes + conditional HTML

- AGENTS.md (root): marks core-ui as optional in the package table and
  boundary rules; points per-package docs to the .hbs template
- apps/storybook/AGENTS.md: rewrites around no-core-ui-by-default;
  stories glob and globals.css import described as post-scaffold steps
- apps/web-next/AGENTS.md: cross-reference updated to template file
- apps/web-tanstack/AGENTS.md: cross-reference updated to template file
- docs/architecture/data-flow-explainer.html: core-ui bullet notes
  optional status + generator command

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

apps/web-next/AGENTS.md

@@ -108,4 +108,4 @@ Run: `pnpm test:e2e` starts the dev server and runs all `.spec.ts` files.
 - **Feature packages:** `packages/{auth,blog,media,marketing-pages,navigation}/`
 - **tRPC composition:** `packages/core-api/AGENTS.md`
 - **tRPC client + provider (optional):** scaffold `@repo/core-trpc` first, then see `turbo/generators/templates/core-package/trpc/AGENTS.md.hbs`
-- **UI components (optional):** scaffold `@repo/core-ui` first, then see `packages/core-ui/AGENTS.md`
+- **UI components (optional):** scaffold with `pnpm turbo gen core-package ui`, then see `turbo/generators/templates/core-package/ui/AGENTS.md.hbs`