Description

From TASK-008 description (deferred, not an AC there): toggle the checkbox on the current list line between [ ] and [x] with a keybind, without hand-editing the brackets. Should work whether or not the cursor is on the brackets; no-op on non-checkbox lines.

Acceptance Criteria

• #1 A keybind toggles the current line's checkbox [ ] <-> [x]
• #2 No-op on lines without a checkbox
• #3 README/help document the key