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>

package.json

@@ -11,6 +11,7 @@
     "preinstall": "node -e \"var v = +process.versions.node.split('.')[0]; if (v < 22) { console.error('Node 22 or newer required (got ' + process.versions.node + '). Run: nvm use 22'); process.exit(1); }\"",
     "gen:index": "bun run ./scripts/gen-index.ts",
     "fix:env": "bun run ./scripts/fix-env.ts",
+    "todo:index": "bun run ./scripts/todo-index.ts",
     "nudge": "bun run --env-file=.env ./scripts/nudge-agent.ts",
     "doctor": "bun run --env-file=.env ./scripts/doctor.ts",
     "secret:set": "bun run --env-file=.env ./scripts/secret-set.ts",