feat(mcp/external): add ListTags + inject TagRepository

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -10,6 +10,8 @@ namespace ClaudeDo.Worker.External;
 
 public sealed record TaskListDto(string Id, string Name, string? WorkingDir);
 
+public sealed record TagDto(long Id, string Name);
+
 public sealed record TaskDto(
     string Id,
     string ListId,
@@ -29,17 +31,20 @@ public sealed class ExternalMcpService
     private readonly ListRepository _lists;
     private readonly QueueService _queue;
     private readonly HubBroadcaster _broadcaster;
+    private readonly TagRepository _tags;
 
     public ExternalMcpService(
         TaskRepository tasks,
         ListRepository lists,
         QueueService queue,
-        HubBroadcaster broadcaster)
+        HubBroadcaster broadcaster,
+        TagRepository tags)
     {
         _tasks = tasks;
         _lists = lists;
         _queue = queue;
         _broadcaster = broadcaster;
+        _tags = tags;
     }
 
     [McpServerTool, Description("List all task lists available in ClaudeDo.")]
@@ -183,6 +188,13 @@ public sealed class ExternalMcpService
         return cancelled;
     }
 
+    [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)
+    {
+        var tags = await _tags.GetAllAsync(cancellationToken);
+        return tags.Select(t => new TagDto(t.Id, t.Name)).ToList();
+    }
+
     private static TaskDto ToDto(TaskEntity t) => new(
         t.Id,
         t.ListId,