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>

docs/architecture/data-flow-explainer.html

@@ -2284,7 +2284,7 @@ footer .colophon {
         <div class="pros"><h5>Pros</h5><ul>
           <li>Apps depend on the feature's UI surface explicitly — clear separation from contracts</li>
           <li>Query builders sit next to the data they query — refactoring is local</li>
-          <li>Components for feature-specific UI (e.g. <code>ArticleCard</code>) live with the feature, not in <code>core-ui</code></li>
+          <li>Components for feature-specific UI (e.g. <code>ArticleCard</code>) live with the feature, not in <code>core-ui</code> (<em>optional package — scaffold via <code>pnpm turbo gen core-package ui</code></em>)</li>
         </ul></div>
         <div class="cons"><h5>Cons</h5><ul>
           <li>Features without UI today (auth, media) still need a placeholder <code>export {}</code></li>