feat(sidebar): add SidebarContext for composable sidebar

Create context and hook to share collapsed state and toggle callback
between compound component sub-components.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/design-system/layout/Sidebar/SidebarContext.ts

@@ -0,0 +1,14 @@
+import { createContext, useContext } from 'react'
+
+export interface SidebarContextValue {
+  collapsed: boolean
+  onCollapseToggle?: () => void
+}
+
+export const SidebarContext = createContext<SidebarContextValue>({
+  collapsed: false,
+})
+
+export function useSidebarContext(): SidebarContextValue {
+  return useContext(SidebarContext)
+}