feat(mcp/external): add SetTaskTags

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -231,6 +231,23 @@ public sealed class ExternalMcpService
         await _broadcaster.TaskUpdated(taskId);
     }
 
+    [McpServerTool, Description("Replace the full tag set on an existing task. Missing tag names auto-create. Refuses if the task is Running.")]
+    public async Task<TaskDto> SetTaskTags(
+        string taskId,
+        IReadOnlyList<string> tags,
+        CancellationToken cancellationToken)
+    {
+        var task = await _tasks.GetByIdAsync(taskId, cancellationToken)
+            ?? throw new InvalidOperationException($"Task {taskId} not found.");
+        if (task.Status == TaskStatus.Running)
+            throw new InvalidOperationException("Cannot retag a running task. Cancel it first.");
+
+        await _tasks.SetTagsAsync(taskId, tags, cancellationToken);
+        var reload = (await _tasks.GetByIdAsync(taskId, cancellationToken))!;
+        await _broadcaster.TaskUpdated(taskId);
+        return ToDto(reload);
+    }
+
     [McpServerTool, Description("List all known tags. Useful for discovering existing tag names (including 'agent' which marks tasks for auto-execution) before tagging.")]
     public async Task<IReadOnlyList<TagDto>> ListTags(CancellationToken cancellationToken)
     {