feat(website): add /categories/built-in/ page for Built-in tag filter

Register Built-in as a navigable filter path alongside regular category
and group slugs, emit the page during build, add it to the sitemap, and
wire the Built-in tag buttons in index.html and category.html to navigate
there via data-url.

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

website/templates/index.html

@@ -247,7 +247,11 @@
               {{ entry.groups[0] }}
             </button>
             {% if entry.source_type == 'Built-in' %}
-            <button class="tag tag-source" data-value="Built-in">
+            <button
+              class="tag tag-source"
+              data-value="Built-in"
+              data-url="/categories/built-in/"
+            >
               Built-in
             </button>
             {% endif %}