Move TODO to issue tracker

I keep forgetting the TODO file exists, and anyway it's an ineffective place to track TODOs relative to a web-based issue tracker, so I think we should migrate its contents here.

See also #43 but moving the issue tracker elsewhere is an orthogonal problem.

Start by locating and reading the TODO file, then review issue #43 for the related issue-tracker context. Migrate the TODO contents into separate tracker issues as appropriate and confirm the repository no longer relies on the TODO file for tracking work.

Written by the indexing model from the issue text.