docs: tighten wording, drop the planning scratch files

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.

src/lib/undo.js

@@ -24,7 +24,7 @@ export function createUndoStack(limit = 10) {
  * Reverses one entry against the current task list.
  *
  * Indices are clamped because the list may have changed since the entry was
- * recorded — a task deleted from position 5 can be restored into a list that
+ * recorded. A task deleted from position 5 can be restored into a list that
  * has since shrunk to two items, and landing at the end beats throwing.
  *
  * `clearCompleted` entries must record `removed` in ascending index order, so