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/email/_index.md

@@ -0,0 +1,11 @@
+---
+theme: email
+heading: "Email"
+kind: theme
+---
+
+Email became a plugin on 2026-08-18 (`plugins/email/`, `gitea.officer.dev/plugins/email`). Its work queue
+went with it — the open items pointed at `sidecar/email/store.ts`, a path in another repository, which is
+exactly the way a leftover list sends someone to edit files that are not there.
+
+Anything still owed to the mail client belongs in that repository's own TODO.