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/rollover.js

@@ -7,7 +7,7 @@ import { toKey } from './dates.js'
  * today (oldest day first, within-day order preserved, ahead of anything
  * already in today), completed tasks are discarded, and the old key is
  * deleted. Pruning is therefore a side effect of carrying, so there is no
- * separate retention policy. Future keys — Tomorrow — are never touched.
+ * separate retention policy. Future keys, meaning Tomorrow, are never touched.
  *
  * `now` is a parameter rather than a `new Date()` call so the boundary is
  * testable. This replaces `checkAndMigrateTasks`, whose guard compared a
@@ -17,7 +17,7 @@ export function rollover(storage, now) {
   const todayKey = toKey(now)
 
   // listTaskKeys yields only well-formed ISO keys, which is what makes this
-  // string comparison safe — and is why the key format changed.
+  // string comparison safe, and is why the key format changed.
   const pastKeys = storage
     .listTaskKeys()
     .filter((key) => key < todayKey)

src/lib/shortcuts.js

@@ -4,10 +4,10 @@ const TEXT_FIELDS = new Set(['INPUT', 'TEXTAREA'])
  * Whether a keydown should trigger task undo.
  *
  * The subtlety is the text field. Deferring to a focused field's own undo
- * sounds right, but the add-task input is where focus normally sits — you
+ * sounds right, but the add-task input is where focus normally sits. You
  * click it to add a task and focus stays there, and on macOS clicking a
  * button does not move focus. Guarding on focus alone therefore disables
- * undo in precisely the situation it is needed: right after deleting a task.
+ * undo in the one situation it is needed: right after deleting a task.
  *
  * So it defers only when the field actually holds text worth undoing. An
  * empty input has nothing for the browser to restore, and task undo wins.

src/lib/tasks.js

@@ -1,5 +1,5 @@
 /** crypto.randomUUID requires a secure context, and Negotium over plain HTTP
- *  on a LAN is a real self-hosted deployment shape — hence the fallback. */
+ *  on a LAN is a real deployment shape for this app, hence the fallback. */
 function newId() {
   if (typeof crypto !== 'undefined' && typeof crypto.randomUUID === 'function') {
     return crypto.randomUUID()

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