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/session/GitHubIntegrationDialog.tsx

@@ -284,7 +284,7 @@ export function GitHubIntegrationDialog({
   const isGitHubConnected = githubAuthChecked && githubAuthStatus?.connected === true;
 
   const openGitHubSettings = () => {
-    setSettingsPage('github');
+    setSettingsPage('integrations');
     setSettingsDialogOpen(true);
   };
 

packages/ui/src/components/session/GitHubIssuePickerDialog.tsx

@@ -230,7 +230,7 @@ export function GitHubIssuePickerDialog({
   const repoUrl = result?.repo?.url ?? null;
 
   const openGitHubSettings = React.useCallback(() => {
-    setSettingsPage('github');
+    setSettingsPage('integrations');
     setSettingsDialogOpen(true);
   }, [setSettingsDialogOpen, setSettingsPage]);
 

packages/ui/src/components/session/GitHubPrPickerDialog.tsx

@@ -217,7 +217,7 @@ export function GitHubPrPickerDialog({
   const connected = githubAuthChecked ? result?.connected !== false : true;
 
   const openGitHubSettings = React.useCallback(() => {
-    setSettingsPage('github');
+    setSettingsPage('integrations');
     setSettingsDialogOpen(true);
   }, [setSettingsDialogOpen, setSettingsPage]);