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/multi-user/remove-dead-username-plumbing.md

@@ -0,0 +1,14 @@
+---
+id: multi-user/remove-dead-username-plumbing
+title: "Remove dead `username` plumbing"
+theme: multi-user
+status: open
+created: 2026-08-20
+date_source: assumed
+order: 20
+---
+
+**Remove dead `username` plumbing.** `send-claude-code.ts` declares `username` in two types
+without using it. (`toShellUsername` is NOT dead — `server.tsx:190` and
+`pipeline-job-manager.ts:267` both call it. The `provision.ts` caller this item used to name no
+longer exists.)