Add weeknotes system with first entry

- Add weeknotes landing page with auto-listing of all entries
- Create first weeknote: December 20–27, 2025
- Add Weeknotes to secondary menu alongside Now/Then/Future
- Create custom layout to display all weeknotes chronologically
- Update weeknotes command to use date ranges and current time
- Credit Genehack for weeknotes inspiration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

layouts/weeknotes/single.html

@@ -0,0 +1,24 @@
+{{ define "main" }}
+  <article class="post">
+    <header class="post-header">
+      <h1 class="post-title">{{ .Title }}</h1>
+    </header>
+
+    <div class="post-content">
+      {{ .Content }}
+    </div>
+
+    <h2>All Weeknotes</h2>
+    <ul class="posts-list">
+      {{ range (where .Site.RegularPages "Params.tags" "intersect" (slice "weeknotes")).ByDate.Reverse }}
+        <li class="posts-list-item">
+          <a class="posts-list-item-title" href="{{ .Permalink }}">{{ .Title }}</a>
+          <span class="posts-list-item-description">
+            {{ partial "icon.html" (dict "ctx" $ "name" "calendar") }}
+            {{ .PublishDate.Format "Jan 2, 2006" }}
+          </span>
+        </li>
+      {{ end }}
+    </ul>
+  </article>
+{{ end }}