Organize columns by subject tags; add /subjects/ page

- Drive the columns collection from the `column` tag, recovering 3 columns
  authored under Notes/ that the glob missed
- Tag the 6 columns with subject tags; add a subjects collection and a
  /subjects/ listing page (newest-first), linked from the footer

.eleventy.js

@@ -69,12 +69,27 @@ module.exports = function (eleventyConfig) {
   eleventyConfig.addShortcode("image", imageShortcode);
   eleventyConfig.addShortcode("heroImage", heroImageShortcode);
 
+  // A "column" is anything tagged `column`, wherever it lives on disk
+  // (some columns are authored under Notes/). Newest first.
   eleventyConfig.addCollection("columns", function (collectionApi) {
     return collectionApi
-      .getFilteredByGlob("columns/**/*.md")
+      .getFilteredByTag("column")
       .sort((a, b) => b.date - a.date);
   });
 
+  // The subject tags actually in use across columns — everything except the
+  // `column` type marker and Eleventy's reserved `all`. Sorted for stable order.
+  eleventyConfig.addCollection("subjects", function (collectionApi) {
+    const reserved = new Set(["all", "column"]);
+    const subjects = new Set();
+    for (const item of collectionApi.getFilteredByTag("column")) {
+      for (const tag of item.data.tags || []) {
+        if (!reserved.has(tag)) subjects.add(tag);
+      }
+    }
+    return [...subjects].sort();
+  });
+
   eleventyConfig.addFilter("inlineMarkdown", function (text) {
     return text.replace(/~~(.+?)~~/g, "<del>$1</del>");
   });

Notes/do-people-think.md

@@ -2,7 +2,7 @@
 title: Do ~~Machines~~ People Think?
 date: 2026-04-02
 layout: column.njk
-tags: column
+tags: [column, philosophy]
 permalink: /columns/2026/do-people-think/
 ---
 

Notes/how-to-write.md

@@ -2,7 +2,7 @@
 title: How To Write
 date: 2026-04-06
 layout: column.njk
-tags: column
+tags: [column, writing]
 permalink: /columns/2026/how-to-write/
 ---
 

Notes/systems.md

@@ -2,7 +2,7 @@
 title: systems
 date: 2026-04-06
 layout: column.njk
-tags: column
+tags: [column, philosophy]
 permalink: /columns/2026/systems/
 ---
 

_includes/base.njk

@@ -25,6 +25,7 @@
     <nav class="footer-nav">
       <a href="/about/">about</a>
       <a href="/code/">code</a>
+      <a href="/subjects/">subjects</a>
     </nav>
   </footer>
 </body>

_includes/column.njk

@@ -38,6 +38,7 @@
     <nav class="footer-nav">
       <a href="/about/">about</a>
       <a href="/code/">code</a>
+      <a href="/subjects/">subjects</a>
     </nav>
   </footer>
 </body>

columns/2026-04-19-happy-idiots.md

@@ -2,7 +2,7 @@
 title: Happy Idiots
 date: 2026-04-19
 layout: column.njk
-tags: column
+tags: [column, society]
 permalink: /columns/2026/happy-idiots/
 ---
 

columns/2026-05-14-thrilling.md

@@ -2,7 +2,7 @@
 title: 12-Watt Bulbs 
 date: 2026-05-14
 layout: column.njk
-tags: column
+tags: [column, society]
 permalink: /columns/2026/thrilling/
 ---
 

columns/2026-06-23-hamming-it-up.md

@@ -2,7 +2,7 @@
 title: Hamming It Up
 date: 2026-06-23
 layout: column.njk
-tags: column
+tags: [column, computing]
 permalink: /columns/2026/hamming/
 ---
 

css/style.css

@@ -175,6 +175,33 @@ a:hover {
   margin: 2rem 0;
 }
 
+/* subjects index */
+
+.subject-group {
+  margin-bottom: 2rem;
+}
+
+.subject-group h2 {
+  font-size: 1.1rem;
+  margin-bottom: 0.5rem;
+}
+
+.subject-group ul {
+  list-style: none;
+  padding-left: 0;
+}
+
+.subject-group li {
+  margin-bottom: 0.3rem;
+}
+
+.subject-group time {
+  font-family: var(--font-ui);
+  font-size: 0.8rem;
+  color: var(--color-muted);
+  margin-left: 0.5rem;
+}
+
 /* sidebar */
 
 .sidebar {

subjects.njk

@@ -0,0 +1,23 @@
+---
+layout: base.njk
+title: Subjects
+permalink: /subjects/
+---
+
+<h1>Subjects</h1>
+
+{%- for subject in collections.subjects %}
+<section class="subject-group">
+  <h2>{{ subject }}</h2>
+  <ul>
+    {%- for post in collections[subject] | sort(true, false, "date") %}
+    <li>
+      <a href="{{ post.url }}">{{ post.data.title | inlineMarkdown | safe }}</a>
+      <time datetime="{{ post.date | isoDate }}">{{ post.date | readableDate }}</time>
+    </li>
+    {%- endfor %}
+  </ul>
+</section>
+{%- else %}
+<p>No subjects yet.</p>
+{%- endfor %}