fix(admin): surface coach-feedback in sidebar nav

The coach-feedback slug was registered in ADMIN_SECTION_SLUGS and the
page rendered correctly via /admin/[section], but the entry was never
added to the hand-maintained ADMIN_SECTIONS array. Operators could only
reach the view by typing the URL directly. Add the entry next to
ai-quality so the two AI-quality views sit together, and add a drift
guard test that walks ADMIN_SECTION_SLUGS and asserts every slug has a
matching nav entry. Same drift class would recur on the next admin
section addition without the guard.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/admin/admin-shell.tsx

@@ -75,6 +75,11 @@ export const ADMIN_SECTIONS: readonly AdminSection[] = [
     titleKey: "admin.section.ai-quality.title",
     icon: Sparkles,
   },
+  {
+    slug: "coach-feedback",
+    titleKey: "admin.section.coach-feedback.title",
+    icon: Sparkles,
+  },
   {
     slug: "feedback",
     titleKey: "admin.section.feedback.title",