fix(git): resolve blank nested-repo surfaces; add repository pickers

The resolution gate on the pull-request, walkthrough, and mobile changes
surfaces stayed on forever (rootIsGitRepo stays false on a non-repo
root) while NestedRepoResolutionStates exits once the selected
repository probes as a repository, so those surfaces rendered nothing.
The gate now shows resolution states only while the operating directory
has not proven to be a repository, matching GitView.

Extract GitHeader's repository switcher into git/NestedRepoPicker and
mount it in the diff toolbar, a new slim header in the pull-request
view, the walkthrough header, and the mobile changes header. The pick
is shared per root, so every surface follows.

The walkthrough tab mounts keep-alive and hidden; it now receives a
visible prop so discovery waits until the tab is actually opened. Add
component tests for the shared resolution states.

packages/ui/src/components/layout/ContextPanel.tsx

@@ -1228,7 +1228,7 @@ export const ContextPanel: React.FC = () => {
         {hasWalkthroughTab ? (
           <div className={cn('absolute inset-0', activeTab?.mode === 'walkthrough' ? 'block' : 'hidden')}>
             <React.Suspense fallback={null}>
-              <WalkthroughView directory={effectiveDirectory} />
+              <WalkthroughView directory={effectiveDirectory} visible={activeTab?.mode === 'walkthrough'} />
             </React.Suspense>
           </div>
         ) : null}