feat(settings): add item search (#1592)

Adds item-level search inside Settings so users can find concrete settings like provider auth, agent mode, terminal font size, tunnel options, notification events, and similar controls instead of only filtering top-level pages.
Groups search results by Settings page and shows localized labels plus optional descriptions where useful.
Supports keyboard navigation with Arrow Up/Down, Enter, and Escape, matching the existing autocomplete interaction style.
Opens the correct Settings page or split-page draft state before scrolling to the matching control.
Highlights the matched setting with a subtle token-based background so users can see where they landed without an aggressive outline.
Adds explicit data-settings-item anchors across Settings pages and a centralized search registry with runtime/mobile availability guards.
Updates Settings UI skill guidance so future Settings changes keep search registry entries, anchors, localization, and availability guards in sync.

packages/ui/src/components/sections/agents/AgentsSidebar.tsx

@@ -334,6 +334,7 @@ export const AgentsSidebar: React.FC<AgentsSidebarProps> = ({ onItemSelect }) =>
         <div className="flex items-center justify-between gap-2">
           <span className="typography-meta text-muted-foreground">{t('settings.agents.sidebar.total', { count: visibleAgents.length })}</span>
           <Button size="sm"
+            data-settings-item="agents.create"
             variant="ghost"
             className="h-7 w-7 px-0 -my-1 text-muted-foreground"
             onClick={handleCreateNew}