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.

tests/ClaudeDo.Worker.Tests/Lifecycle/AttachmentOrphanRecoveryTests.cs

@@ -3,6 +3,7 @@ using ClaudeDo.Data.Models;
 using ClaudeDo.Worker.Lifecycle;
 using ClaudeDo.Worker.Tests.Infrastructure;
 using Microsoft.Extensions.Logging.Abstractions;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
 
 namespace ClaudeDo.Worker.Tests.Lifecycle;