feat(attachments): inject reference files into the run + clean up files on delete

TaskRunner appends attached files (absolute paths) to the run prompt as the
read-only Reference files section. Task and list deletes now remove the
on-disk attachment dir eagerly, and a startup AttachmentOrphanRecovery sweep
drops any attachments/<taskId>/ whose task no longer exists (covers list
cascade and planning-discard paths).

tests/ClaudeDo.Worker.Tests/External/AddSubtaskToolTests.cs

@@ -69,7 +69,7 @@ public sealed class AddSubtaskToolTests : IDisposable
         var argsBuilder = new ClaudeArgsBuilder();
         var state = TaskStateServiceBuilder.Build(dbFactory).State;
         var runner = new TaskRunner(fake, dbFactory, broadcaster, wtManager, argsBuilder, cfg,
-            NullLogger<TaskRunner>.Instance, state, new TaskRunTokenRegistry());
+            NullLogger<TaskRunner>.Instance, state, new TaskRunTokenRegistry(), new AttachmentStore());
         var waker = new ClaudeDo.Worker.Queue.QueueWaker();
         var picker = new ClaudeDo.Worker.Queue.QueuePicker(dbFactory);
         var overrideSlot = new OverrideSlotService(dbFactory, runner, NullLogger<OverrideSlotService>.Instance);