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/Views/TaskEditorView.axaml.cs

@@ -1,4 +1,7 @@
 using Avalonia.Controls;
+using Avalonia.Interactivity;
+using Avalonia.Platform.Storage;
+using ClaudeDo.Ui.ViewModels;
 
 namespace ClaudeDo.Ui.Views;
 
@@ -8,4 +11,19 @@ public partial class TaskEditorView : Window
     {
         InitializeComponent();
     }
+
+    private async void OnBrowseAgent(object? sender, RoutedEventArgs e)
+    {
+        var files = await StorageProvider.OpenFilePickerAsync(new FilePickerOpenOptions
+        {
+            Title = "Select Agent File",
+            AllowMultiple = false,
+            FileTypeFilter = new[] { new FilePickerFileType("Agent Files") { Patterns = ["*.md"] } },
+        });
+        if (files.Count == 0) return;
+        var path = files[0].TryGetLocalPath();
+        if (path is null) return;
+        if (DataContext is TaskEditorViewModel vm)
+            vm.SetAgentFromPath(path);
+    }
 }