feat(worker): accept #123 task numbers as MCP tool input

TaskIdResolver resolves a #123/bare-123 taskId parameter to its GUID
before any lookup, across every External/ MCP tool that takes a task
id, including the batch tools' id arrays (via delegation to the
already-resolving single-entity methods) and update_task's
dependsOnTaskId (empty string still passes through unchanged as the
clear-link sentinel). An unknown number throws a clear error instead
of a silent null. McpToolDocs.TaskNumberHint tells the agent to refer
to tasks as #<number> when reporting to the user, added to the
description of get_task, list_tasks, add_task, update_task_status and
review_task.

src/ClaudeDo.Worker/External/HandoffMcpTools.cs

@@ -40,6 +40,9 @@ public sealed class HandoffMcpTools
         if (survivingTaskIds.Count == 0)
             throw new InvalidOperationException("survivingTaskIds must contain at least one task id.");
 
+        taskId = await TaskIdResolver.ResolveAsync(_tasks, taskId, cancellationToken);
+        survivingTaskIds = await TaskIdResolver.ResolveManyAsync(_tasks, survivingTaskIds, cancellationToken);
+
         var task = await _tasks.GetByIdAsync(taskId, cancellationToken)
             ?? throw new InvalidOperationException($"Task {taskId} not found.");