feat: add category pages and per-category feeds to site builder

Extend the publishing pipeline to generate category index pages,
per-category archive pages, per-category RSS/JSON feeds, and display
categories on individual post pages and post lists.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

layouts/simplecss/categories_single.html

@@ -0,0 +1,16 @@
+<h2>{{ .Category.Name }}</h2>
+{{ if .DescriptionHTML }}
+  <div>{{ .DescriptionHTML }}</div>
+{{ end }}
+{{ range .Posts }}
+  {{ if .Post.Title }}<h3>{{ .Post.Title }}</h3>{{ end }}
+  {{ .HTML }}
+  <a href="{{ url_abs .Path }}">{{ format_date .Post.PublishedAt }}</a>
+  {{ if .Categories }}
+    <p>
+    {{ range .Categories }}
+      <a href="{{ url_abs (printf "/categories/%s" .Slug) }}">{{ .Name }}</a>
+    {{ end }}
+    </p>
+  {{ end }}
+{{ end }}