todo: one file per item, grouped by theme, with a generated index

TODO.md had reached 1050 lines and 72 open items across 28 sections, read mostly
on a phone. It is now a directory: todo/<theme>/<item>.md, one card per file, with
frontmatter carrying id, status, created date and order, and the prose body copied
across verbatim. TODO.md becomes a generated index (bun run todo:index).

Dates: where an item's prose stated one it is used; otherwise its old section's;
otherwise 2026-08-20, floored at the day before the split rather than invented.
date_source records which.

Shaped for a two-way Kanban board, filed as todo/tooling — a card is a file, id is
the identity, frontmatter is the machine's half and the body is the owner's.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

todo/tooling/_index.md

@@ -0,0 +1,8 @@
+---
+theme: tooling
+heading: "Tooling — the list about the list"
+kind: theme
+---
+
+Work on how the todos themselves are captured, stored and read. The list is now a filing system
+(`todo/README.md`), which makes it something that can have its own defects and its own features.