Fix Hugo build: nav.html used invalid hugo.Data syntax, fix deploy.sh path

nav.html: hugo.Data.nav.items → .Site.Data.nav.items (hugo.Data is not
a valid accessor for data files in any Hugo version).

deploy.sh: run hugo from site/ subdir, output to public/ at repo root
where nginx expects it. Use /usr/local/bin/hugo (v0.147.4) explicitly.

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

site/layouts/partials/nav.html

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