Add light border radius across components ...

- New tokens: $border-radius-sm (3px controls), $border-radius-md
  (6px panels), $border-radius-pill (switch track)
- Apply via hard_panel and per-component: buttons, inputs, tabs,
  range thumb, checkbox, switch, chips, avatars, dropdown/popover/
  tooltip panels, modal, toast, file upload, input group, code examples
- Drawer stays sharp (flush to screen edge)

Expose design tokens as --gn-* CSS custom properties in kit.css

Add AI agent guide, AGENTS.md and useWhen to component catalog