fix(ui): make Files-tab system prompts view-only, drop external-editor edit path

Editing a prompt file (even externally via "Open in editor") freezes it as a
user customization and blocks future default updates. The Files tab no longer
opens an external editor; it shows each prompt's content read-only in-app via
a new ViewPromptCommand, dropping the file-seeding side effect. Reset to
default and the customized-prompts deviation list are unchanged.

src/ClaudeDo.Ui/ViewModels/Modals/Settings/FilesSettingsTabViewModel.cs

@@ -1,5 +1,4 @@
 using System.Collections.ObjectModel;
-using System.Diagnostics;
 using ClaudeDo.Data;
 using ClaudeDo.Ui.Localization;
 using ClaudeDo.Ui.Services;
@@ -15,6 +14,8 @@ public sealed partial class FilesSettingsTabViewModel : ViewModelBase
     [ObservableProperty] private string _statusMessage = "";
     [ObservableProperty] private bool _isBusy;
     [ObservableProperty] private bool _hasCustomizedPrompts;
+    [ObservableProperty] private string? _viewedKindName;
+    [ObservableProperty] private string _viewedContent = "";
 
     public string SystemPromptPath { get; } = PromptFiles.PathFor(PromptKind.System);
     public string PlanningPromptPath { get; } = PromptFiles.PathFor(PromptKind.Planning);
@@ -49,20 +50,11 @@ public sealed partial class FilesSettingsTabViewModel : ViewModelBase
     }
 
     [RelayCommand]
-    private void OpenPrompt(string? kindName)
+    private void ViewPrompt(string? kindName)
     {
         if (!Enum.TryParse<PromptKind>(kindName, ignoreCase: true, out var kind)) return;
-        try
-        {
-            // No override file yet: seed it with today's bundled default (hash-tracked, so a later
-            // default change reconciles this file away automatically unless the user actually edits it)
-            // rather than leaving a plain file that would freeze the prompt forever.
-            if (!File.Exists(PromptFiles.PathFor(kind)))
-                PromptFiles.Save(kind, PromptFiles.DefaultFor(kind));
-            Process.Start(new ProcessStartInfo(PromptFiles.PathFor(kind)) { UseShellExecute = true });
-        }
-        catch (Exception ex) { StatusMessage = Loc.T("vm.filesTab.openFailed", ex.Message); }
-        finally { RefreshCustomizedPrompts(); }
+        ViewedKindName = kindName;
+        ViewedContent = PromptFiles.ReadOrDefault(kind);
     }
 
     private void RefreshCustomizedPrompts()