feat(ui): agent config inline in detail panel, file picker, subtask UI

TaskDetailView now edits Model / SystemPrompt / Agent inline (LostFocus
save), matching the modal editor. Both TaskEditorView and TaskDetailView
gain a Browse button that opens a .md file picker — external agent
paths are preserved on reload via a synthetic AgentInfo entry. Both
views also render the per-task subtask checklist (CheckBox + TextBox +
remove), with diff-on-save in the editor and inline-save in the detail
panel.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/ViewModels/TaskListViewModel.cs

@@ -194,7 +194,7 @@ public partial class TaskListViewModel : ViewModelBase
         var taskTags = await _taskRepo.GetTagsAsync(entity.Id);
         var editor = _editorFactory();
         await editor.LoadAgentsAsync(_worker);
-        editor.InitForEdit(entity, taskTags);
+        await editor.InitForEditAsync(entity, taskTags);
 
         var window = new TaskEditorView { DataContext = editor };
         editor.RequestClose += () => window.Close();