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/git-identities/GitPage.tsx

@@ -117,10 +117,12 @@ export const GitPage: React.FC = () => {
     <>
       <ScrollableOverlay outerClassName="h-full" className="w-full bg-background">
         <div className="mx-auto w-full max-w-3xl space-y-6 p-3 sm:p-6 sm:pt-8">
-          <GitHubSettings />
+          <div data-settings-item="git.github-account">
+            <GitHubSettings />
+          </div>
 
           {/* Identities Section */}
-          <div className="border-t border-border/40 pt-6">
+          <div data-settings-item="git.identities" className="border-t border-border/40 pt-6">
             <div className="mb-3 px-1 flex items-start justify-between gap-4">
               <div className="flex items-center gap-2">
                 <h3 className="typography-ui-header font-semibold text-foreground">{t('settings.gitIdentities.page.section.title')}</h3>