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/hooks/useKeyboardShortcuts.ts

@@ -30,6 +30,7 @@ import { readEmbeddedThemeSearchParams } from '@/contexts/theme-embedded-bootstr
 import { useDirectoryStore } from '@/stores/useDirectoryStore';
 import { useFeatureFlagsStore } from '@/stores/useFeatureFlagsStore';
 import { useProjectsStore } from '@/stores/useProjectsStore';
+import { useGitHubAuthStore } from '@/stores/useGitHubAuthStore';
 import { useLinearAuthStore } from '@/stores/useLinearAuthStore';
 import { useEffectiveDirectory } from '@/hooks/useEffectiveDirectory';
 import { getCycledPrimaryAgentName } from '@/components/chat/mobileControlsUtils';
@@ -507,6 +508,7 @@ export const useKeyboardShortcuts = () => {
             screenWidth: window.innerWidth,
             tabs: panel?.tabs ?? [],
             linearConnected: useLinearAuthStore.getState().status?.connected === true,
+            githubConnected: useGitHubAuthStore.getState().status?.connected === true,
           });
           const target = visibleSurfaces[switchSurfaceDigit - 1];
           if (target) {