Fix PR thread upstream tracking for fork branches

- Restore/set upstream tracking when preparing local and reused PR worktrees
- Derive fork `owner/repo` from PR URL when GitHub omits `headRepositoryNameWithOwner`
- Expand GitManager tests to cover local mode, URL-derived fork identity, and reused worktree upstream recovery
268016d872Julius Marminge committed on 3/10/2026, 5:50:45 AM· committed by Julius Marmingeparentac3f22c
2 files changedLine totals unavailable