feat(ui): worktree modal with tree view and M/A badges

Adds WorktreeModalView/ViewModel showing git status --porcelain as a
recursive file tree with M/A/D/? status badges. Wires the Worktree
button in AgentStripView to OpenWorktreeCommand on DetailsIslandViewModel.
Adds GetStatusPorcelainAsync to GitService.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Islands/AgentStripView.axaml.cs

@@ -30,7 +30,6 @@ public partial class AgentStripView : UserControl
                 var owner = TopLevel.GetTopLevel(this) as Window;
                 if (owner == null) return;
                 var modal = new WorktreeModalView { DataContext = worktreeVm };
-                worktreeVm.CloseCommand.Subscribe(_ => modal.Close());
                 await modal.ShowDialog(owner);
             };
         }