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

content/docs/components.md

@@ -93,13 +93,13 @@ Mark a block as a session record with `data-session`. Optional `data-mode` indic
 Internal knowledge-graph links. `data-wikilink` on an anchor renders it with a dotted underline to distinguish it from a regular hyperlink.
 
 ```html
-<a href="/vault/tasks/" data-wikilink>Tasks</a>
+<a href="/notes/tasks/" data-wikilink>Tasks</a>
 ```
 
 In Hugo, use the `wikilink` shortcode:
 
 ```
-{{</* wikilink "Tasks" "/vault/tasks/" */>}}
+{{</* wikilink "Tasks" "/notes/tasks/" */>}}
 ```
 
 ---