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

hugo.toml

@@ -20,18 +20,18 @@ title = 'ASW — Agentic Semantic Web'
   weight = 2
 
 [[menus.main]]
-  name = "Vault"
-  url  = "/vault/"
+  name = "Notes"
+  url  = "/notes/"
   weight = 3
 
 [[menus.main]]
-  name = "Papers"
-  url  = "/papers/"
+  name = "Essays"
+  url  = "/essays/"
   weight = 4
 
 [[menus.main]]
-  name = "Posts"
-  url  = "/posts/"
+  name = "Articles"
+  url  = "/articles/"
   weight = 5
 
 [[menus.main]]
@@ -125,7 +125,7 @@ title = 'ASW — Agentic Semantic Web'
 
 [[menus.docs]]
   name   = "ASW Vocabulary"
-  url    = "/posts/asw-vocabulary/"
+  url    = "/articles/asw-vocabulary/"
   parent = "docs-reference"
   weight = 43