fix: wrap nav data in items key for Decap CMS compatibility

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

site/data/nav.yml

@@ -1,3 +1,4 @@
+items:
   - name: Home
     url: /
     weight: 1

site/layouts/partials/nav.html

@@ -1,7 +1,7 @@
 <nav>
   <ul><li><a href="/"><strong>{{ .Site.Title }}</strong></a></li></ul>
   <ul>
-    {{- range sort hugo.Data.nav "weight" }}
+    {{- range sort hugo.Data.nav.items "weight" }}
     <li><a href="{{ .url }}"{{ if eq (relURL .url) $.RelPermalink }} aria-current="page"{{ end }}>{{ .name }}</a></li>
     {{- end }}
   </ul>