refactor: rename content types to semantic taxonomy

- vault → notes (PKM-exported content)
- posts → articles (short-form, no TOC)
- papers → essays (long-form, with TOC)
- type: post → type: article (posts are just short articles)
- layouts/paper → layouts/essay
- 08a-paper.css → 08a-essay.css
- CSS: fix redundant li resets, remove role="main" from article,
  replace <small> prev/next labels, add console layout
- Update hugo.toml menus, internal URLs, front matter throughout
- Add docs/context.md, docs/css-refactor-plan.md

layouts/_default/single.html

@@ -1,14 +1,13 @@
 {{ define "header" }}
   <header>
-    <hgroup>
-      {{ with .Params.eyebrow }}<p data-text="eyebrow">{{ . }}</p>{{ end }}
-      <h1>{{ .Title }}</h1>
-      {{ with .Description }}<p>{{ . }}</p>{{ end }}
-    </hgroup>
+    {{ with .Type }}<p data-text="eyebrow">{{ . }}</p>{{ end }}
     <p data-text="dim">
       <time datetime="{{ .Date.Format "2006-01-02" }}">{{ .Date.Format "January 2006" }}</time>
       {{ with .Params.author }} · {{ . }}{{ end }}
     </p>
+    {{ with .Params.tags }}
+    <p>{{ range . }}<a href="/tags/{{ . }}" data-text="tag">#{{ . }}</a> {{ end }}</p>
+    {{ end }}
   </header>
 {{ end }}