news-digest: hero brand block links to /index.html (latest edition) from any archive page

Wrapped .masthead-brand in <a href="index.html"> in both digest.html.j2
and archive.html.j2 so the hero is a clickable shortcut to the latest
edition. Useful when reading an archived edition and you want to jump
back to the freshest one without going through the archive list.

CSS: color: inherit + text-decoration: none keeps the visual
identical; hover drops opacity to 0.85 for affordance; focus-visible
gets an accent outline so keyboard nav is discoverable.

stacks/news-digest/templates/archive.html.j2

@@ -10,10 +10,10 @@
 <body>
   <header class="masthead" role="banner">
     <div class="masthead-inner">
-      <div class="masthead-brand">
+      <a class="masthead-brand" href="index.html" title="Latest edition">
         <span class="brand-glyph" aria-hidden="true">◢</span>
         <span class="brand-mark">DAILY DIGEST</span>
-      </div>
+      </a>
       <div class="masthead-edition">
         <span class="edition-stamp">ARCHIVE</span>
         <a class="archive-link" href="index.html">← LATEST</a>