fix(git): exit nested-repo states once resolution succeeds

NestedRepoResolutionStates had no success exit: on a non-repo root
rootIsGitRepo stays false forever, so once repositories were found the
pull-request and walkthrough tabs kept showing 'Checking repository'
even after the selected repository probed as a repository. GitView never
hit this because its call site sits inside its own isGitRepo === false
branch. The component now takes the operating directory's probe and
returns null when it resolves true.

DiffView also still keyed its not-a-repository gate and every diff
fetch off the raw project root, so opening a change from a nested
repository showed 'This directory is not a Git repository'. It now
resolves the nested repository for git data and diff operations while
session-scoped lookups (session messages, review-flow directory) stay
on the root.

packages/ui/src/apps/MobileChangesSurface.tsx

@@ -487,6 +487,7 @@ export const MobileChangesSurface: React.FC<MobileChangesSurfaceProps> = ({ onCl
     return renderListState(
       <NestedRepoResolutionStates
         rootIsGitRepo={rootIsGitRepo}
+        resolvedIsGitRepo={isGitRepo}
         nestedRepos={nestedRepos}
         onRetryDiscovery={() => {
           if (rootDirectory) void ensureNestedRepos(rootDirectory, { force: true });