todo lists are frustrating indeed. i use similar system to what you just described, but i keep "next" on the top:

$ head -1 todo.md

gives me next todo item to work on.

to add items i just edit file in vi:

$ vi todo.md

bonus: i keep todo items blocked by other people in another file:

$ vi wait.md

and completed items moved to done.md stamped with date

$ echo "$(date +%Y%m%d) $(head -1 todo.md)" >> done.md
$ sed -i .bak '1d' todo.md

bonus 2: send a copy to my phone

$ rsync -litr ~/src/todo/ phone:storage/shared/src/todo/