* maint: clean up TODO tags

Cleans up existing TODO tags, particularly those that referenced
old Linear issues. Updates the todo-checker script to allow
<#issue> as well as <issue>.

* feat(ci): add todo-issues check to main CI

Updates the main CI workflow to run the todo-issues check. For
now this check will ignore any bare TODOs. We can update the
script later to reject bare TODOs.