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

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/
 ---