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/lib/surfaces/DOCUMENTATION.md

@@ -27,7 +27,9 @@ edge (`components/layout/ContextPanelRail.tsx`) and rendered by
   configure button — `ContextRailSurfacesDialog`), drops the plan surface
   unless plan mode is enabled,
   drops the walkthrough on VS Code and below `WALKTHROUGH_MIN_WIDTH`, hides
-  Linear unless a workspace is connected, and hides `has-content` surfaces
+  Linear unless a workspace is connected, hides the pull-request surface
+  unless GitHub is connected (OAuth or `gh` CLI — signed in from Settings →
+  Integrations), and hides `has-content` surfaces
   until a tab of their mode exists. Both consumers use it so the digit shown
   on a rail badge always maps to the same surface the shortcut opens.