* feat(mobile): surface "Link an existing PR" in PR sidebar empty state
The mobile link-PR building blocks (MobileLinkPrForm, linkMobilePr,
parseGitHubPrReference) existed and were tested, but had no entry point —
only unlink was wired up. Add a "Link an existing PR" action to the no-PR
empty state that opens MobileLinkPrForm and refetches the sidebar on
success, mirroring desktop's link flow (GitHub-scoped via worktree.set).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
* Polish mobile PR link empty state
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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Jinwoo-H <jinwoo0825@gmail.com>
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