fix(shell-docs): exclude deprecated tutorials from built-in-agent sidebar

The Built-in Agent sidebar still rendered the deprecated tutorials section
("Tutorial: AI Todo App", "Tutorial: AI Textarea" with step pages) even
after PR CopilotKit#4987 added 301 redirects for /{fw}/tutorials/:path* paths.
The redirects fired on click, but the entries should never have appeared.

Root cause: integrations/built-in-agent/meta.json declared a
"---Tutorials---" section header followed by a "...tutorials" spread.
BIA runs in docs_mode "authored" which routes through buildFrameworkOnlyNav;
that path preserves section headers and recurses through spreads. The
spread descended into integrations/built-in-agent/tutorials/ and
enumerated the AI Todo App and AI Textarea subfolders into the sidebar.

Other frameworks were already safe: buildFrameworkOverridesNav (used by
generated-mode frameworks like langgraph, mastra, google-adk) explicitly
strips section nodes from per-framework override nav.

Fix: remove the "---Tutorials---" section header and "...tutorials"
spread from BIA's parent meta.json. Nothing now references the
tutorials folder from BIA's nav, so the spread handler never recurses
into it and the entries disappear. Two-line minimal change.

Tutorial source MDX under content/docs/integrations/built-in-agent/tutorials/
stays in place (PDX-100 owns the rewrite). The redirect catalog in
lib/seo-redirects.ts is untouched.

Refs PDX-205.

Author: samjulien

showcase/shell-docs/src/content/docs/integrations/built-in-agent/meta.json

@@ -27,8 +27,6 @@
     "copilot-runtime", "custom-agent", "ag-ui",
     "---Premium Features---",
     "...premium",
-    "---Tutorials---",
-    "...tutorials",
     "---Troubleshooting---",
     "...troubleshooting"
   ]