Removes docs/superpowers, which held a design note and an implementation
plan. Those were working notes rather than anything the project needs, and
they don't belong in the repo.
Everything else here is wording: em dashes swapped for ordinary
punctuation across the README and the code comments, and a few sentences
straightened out.
The README described migration that never ran, and listed Backspace as a
delete key.
It now says what actually happens: unfinished work carries across gaps,
completed tasks are cleared with their day, storage prunes itself, and old
keys convert on first open. Adds Cmd/Ctrl+Z and Alt+Arrow, the touch long
press, the screenshot's new path, and how to run the tests.