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>");
   });