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

assets/css/layers/03-components.css

@@ -983,11 +983,6 @@ nav[data-nav="sidebar"] ul {
   font-size: var(--text-sm);
 }
 
-nav[data-nav="sidebar"] ul li {
-  margin: 0;
-  padding: 0;
-}
-
 nav[data-nav="sidebar"] a {
   display: block;
   padding: var(--space-2) var(--space-3);
@@ -1030,11 +1025,6 @@ aside[data-toc] nav ul {
   font-size: var(--text-xs);
 }
 
-aside[data-toc] nav ul li {
-  margin: 0;
-  padding: 0;
-}
-
 aside[data-toc] nav a {
   display: block;
   padding: var(--space-1) var(--space-2);