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
• #4 Mouse click on the checkbox glyph toggles it
• #5 The toggle keybind works with the cursor anywhere on the line (not just on the brackets)

Implementation Notes

Scope expanded per user: toggle by (a) keybind with cursor anywhere on the line, and (b) mouse click on the checkbox glyph. Mouse handling lives in app.go (it owns tea.MouseMsg + margin/scroll->buffer mapping; see MoveToVisual); the toggle mutation itself should be an editor method (e.g. ToggleCheckbox) so it's unit-testable and undoable.