feat(attachments): MCP tools to attach/list/remove task files

AttachmentMcpTools exposes add_task_attachment (text or base64),
list_task_attachments, and remove_task_attachment on the external MCP
endpoint, so an agent can prepare reference files (plans, scripts) on a task
that will run later. Re-attaching the same name overwrites; add/remove refuse
on a running task.

src/ClaudeDo.Data/Repositories/TaskAttachmentRepository.cs

@@ -29,6 +29,12 @@ public sealed class TaskAttachmentRepository
             .FirstOrDefaultAsync(a => a.TaskId == taskId && a.FileName == fileName, ct);
     }
 
+    public async Task UpdateAsync(TaskAttachmentEntity entity, CancellationToken ct = default)
+    {
+        _context.TaskAttachments.Update(entity);
+        await _context.SaveChangesAsync(ct);
+    }
+
     public async Task DeleteAsync(string taskId, string fileName, CancellationToken ct = default)
     {
         await _context.TaskAttachments