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/Models/TaskAttachmentEntity.cs

@@ -0,0 +1,13 @@
+namespace ClaudeDo.Data.Models;
+
+public sealed class TaskAttachmentEntity
+{
+    public required string Id { get; init; }
+    public required string TaskId { get; init; }
+    public required string FileName { get; set; }
+    public long ByteSize { get; set; }
+    public required DateTime CreatedAt { get; init; }
+
+    // Navigation property
+    public TaskEntity Task { get; set; } = null!;
+}