docs: update composable sidebar spec with clarified decisions

Add searchValue prop for controlled input, SidebarContext for
collapsed state propagation, LayoutShell migration plan, and
icon-rail simultaneous callback behavior.

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

docs/superpowers/specs/2026-04-02-composable-sidebar-design.md

@@ -45,6 +45,7 @@ The outer shell. Renders the sidebar frame with an optional search input and col
 <Sidebar
   collapsed={false}
   onCollapseToggle={() => {}}
+  searchValue=""
   onSearchChange={(query) => {}}
   className=""
 >
@@ -134,6 +135,8 @@ v [icon] APPLICATIONS
 
 Header has: chevron (left), icon, label. Chevron rotates on collapse/expand. Active section gets the amber left-border accent (existing pattern). Clicking the header calls `onToggle`.
 
+**Implementation detail:** `Sidebar.Section` and `Sidebar.Header` need to know the parent's `collapsed` state to switch between expanded and icon-rail rendering. The `<Sidebar>` component provides `collapsed` and `onCollapseToggle` via React context (`SidebarContext`). Sub-components read from context — no prop drilling needed.
+
 **Icon-rail click behavior:** In collapsed mode, clicking a section icon fires both `onCollapseToggle` and `onToggle` simultaneously on the same click. The sidebar expands and the section opens in one motion. No navigation occurs — the user is expanding the sidebar to see what's inside, not committing to a destination. They click a tree item after the section is visible to navigate.
 
 ### `<Sidebar.Footer>`