feat(ui): gate the pull-request surface on GitHub, move the account into it, and GitHub sign-in into Integrations

The pull-request rail icon now appears only while GitHub is connected
(OAuth or gh CLI), like Linear; Linear sits after the walkthrough in the
default rail order. The GitHub account avatar and switcher leave the
header for the pull-request panel, where the walkthrough, refresh, and
account controls share one row and one height, and the account stays
visible on the panel's empty state. A manual refresh keeps its spinner on
screen long enough to read as work done.

GitHub sign-in moves from Settings → Git to Settings → Integrations →
Built-in integrations as a card before Linear; search and the connect
buttons follow it.

packages/ui/src/components/sections/integrations/LinearSettings.tsx

@@ -9,7 +9,6 @@ import { openExternalUrl } from '@/lib/url';
 import { useI18n } from '@/lib/i18n';
 import { focusDesktopWindow, isDesktopShell } from '@/lib/desktop';
 import { Icon } from '@/components/icon/Icon';
-import { SettingsSection } from '@/components/sections/shared/SettingsSection';
 import { LinearProjectMapping } from './LinearProjectMapping';
 import { LinearSessionComments } from './LinearSessionComments';
 
@@ -172,13 +171,6 @@ export const LinearSettings: React.FC = () => {
   const expanded = isWaiting || open;
 
   return (
-    <SettingsSection
-      title={t('settings.integrations.firstParty.title')}
-      info={t('settings.integrations.firstParty.info')}
-      divider={false}
-      settingsItem="integrations.first-party"
-      contentClassName="space-y-3"
-    >
       <Collapsible
         open={expanded}
         onOpenChange={(nextOpen) => {
@@ -345,6 +337,5 @@ export const LinearSettings: React.FC = () => {
           </CollapsibleContent>
         </div>
       </Collapsible>
-    </SettingsSection>
   );
 };