README: add recommended keybinds and a Theming section documenting the
bread-theme generator + live-reloaded shared stylesheet. Add
BREAD_DESIGN_SYSTEM.md to the repo (the README links it) and update it to
describe the single-source-of-truth architecture and the migrated apps
(incl. bos-settings).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
