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/SubtaskItemViewModel.cs

@@ -0,0 +1,23 @@
+using ClaudeDo.Data.Models;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace ClaudeDo.Ui.ViewModels;
+
+public partial class SubtaskItemViewModel : ObservableObject
+{
+    [ObservableProperty] private string _title = string.Empty;
+    [ObservableProperty] private bool _completed;
+
+    public string Id { get; set; } = string.Empty;
+    public string? OriginalTitle { get; set; }
+    public bool OriginalCompleted { get; set; }
+
+    public static SubtaskItemViewModel From(SubtaskEntity e) => new()
+    {
+        Id = e.Id,
+        Title = e.Title,
+        Completed = e.Completed,
+        OriginalTitle = e.Title,
+        OriginalCompleted = e.Completed,
+    };
+}