feat(attachments): data layer for task file attachments

TaskAttachmentEntity (+config, cascade FK), TaskAttachmentRepository, and an
AttachmentStore that writes files under ~/.todo-app/attachments/<taskId>/ with
a path-traversal guard and a 5 MB cap. TaskPromptComposer gains an optional
read-only 'Reference files' section. Migration AddTaskAttachments.

src/ClaudeDo.Data/TaskPromptComposer.cs

@@ -9,7 +9,8 @@ namespace ClaudeDo.Data;
 /// </summary>
 public static class TaskPromptComposer
 {
-    public static string Compose(string title, string? description, IEnumerable<(string Title, bool Completed)> subtasks)
+    public static string Compose(string title, string? description, IEnumerable<(string Title, bool Completed)> subtasks,
+        IEnumerable<string>? attachmentPaths = null)
     {
         var sb = new StringBuilder((title ?? "").Trim());
 
@@ -24,6 +25,14 @@ public static class TaskPromptComposer
                 sb.Append("- [ ] ").Append(s.Title).Append('\n');
         }
 
+        var paths = attachmentPaths?.ToList();
+        if (paths is { Count: > 0 })
+        {
+            sb.Append("\n\n## Reference files\nThese files were attached to this task as read-only reference (they live outside the repo). Read them as needed:\n");
+            foreach (var p in paths)
+                sb.Append("- ").Append(p).Append('\n');
+        }
+
         return sb.ToString();
     }
 }