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/08a-essay.css

@@ -0,0 +1,84 @@
+/**
+ * 08-paper.css
+ * Long-form research / essay content type — article[data-paper]
+ * Promoted from lab/hugo-demo paper/single.html inline <style> block.
+ * Part of: Agentic Semantic Web
+ */
+
+/* ── Reading column ─────────────────────────────────────────────────── */
+
+article[data-paper] {
+  max-width: 68ch;
+  margin-inline: auto;
+}
+
+/* ── Centred masthead ───────────────────────────────────────────────── */
+
+article[data-paper] > header {
+  text-align: center;
+  padding-bottom: var(--space-6);
+  margin-bottom: var(--space-6);
+  border-bottom: var(--border-width) solid var(--border-subtle);
+}
+
+article[data-paper] > header h1 {
+  font-size: clamp(1.6rem, 4vw, 2.4rem);
+  font-weight: var(--font-weight-3);
+  letter-spacing: -0.02em;
+  line-height: 1.15;
+  margin-bottom: var(--space-4);
+}
+
+/* ── Abstract / description ─────────────────────────────────────────── */
+
+article[data-paper] > header [data-abstract] {
+  font-size: var(--text-base);
+  color: var(--text-2);
+  max-width: 52ch;
+  margin-inline: auto;
+  margin-bottom: var(--space-4);
+  line-height: var(--leading);
+  font-style: italic;
+}
+
+/* ── Byline ─────────────────────────────────────────────────────────── */
+
+article[data-paper] > header [data-byline] {
+  font-size: var(--text-sm);
+  color: var(--text-3);
+  font-family: var(--font-mono);
+  margin-bottom: var(--space-3);
+}
+
+article[data-paper] > header [data-byline] time {
+  font-variant-numeric: tabular-nums;
+}
+
+/* ── Tags beneath byline ────────────────────────────────────────────── */
+
+article[data-paper] > header nav[data-role="tag-cloud"] {
+  justify-content: center;
+  margin-top: var(--space-3);
+  margin-bottom: 0;
+}
+
+/* ── Footer prev/next ───────────────────────────────────────────────── */
+
+article[data-paper] > footer {
+  display: flex;
+  justify-content: space-between;
+  gap: var(--space-4);
+  margin-top: var(--space-8);
+  padding-top: var(--space-5);
+  border-top: var(--border-width) solid var(--border-subtle);
+  font-size: var(--text-sm);
+}
+
+article[data-paper] > footer a {
+  color: var(--text-2);
+  text-decoration: none;
+}
+
+article[data-paper] > footer a:hover {
+  color: var(--text);
+}