feat: simplify UX — bare numbers toggle, -N removes

[cemheren]

Addresses #2 (too cumbersome, needs too many cell edits).

Changes

Adds shorthand syntax so most todo operations need just one cell edit:

Before (verbose)	After (shorthand)
todo: done 3	todo: 3 (toggles done/undone)
todo: rm 3	todo: -3
todo: add Buy milk	todo: Buy milk (already worked)

The full command syntax still works — this just adds shortcuts on top.

Quick Start (from updated README)

todo: Buy groceries       <- just type it
todo: 1                   <- toggles task #1 done/undone
todo: -1                  <- removes task #1
todo:                     <- shows your list

Closes #2