fix(planning): attach agent tag to chained children for queue pickup

Worker queue picker requires the 'agent' tag — without it children created
through QueueSubtasksSequentiallyAsync sat in 'Queued' forever. Attach the
tag automatically when wiring up the chain.

src/ClaudeDo.Worker/Planning/PlanningChainCoordinator.cs

@@ -19,6 +19,7 @@ public sealed class PlanningChainCoordinator
             ?? throw new InvalidOperationException($"Task {parentTaskId} not found.");
 
         var children = await ctx.Tasks
+            .Include(t => t.Tags)
             .Where(t => t.ParentTaskId == parentTaskId)
             .OrderBy(t => t.SortOrder).ThenBy(t => t.CreatedAt)
             .ToListAsync(ct);
@@ -31,8 +32,14 @@ public sealed class PlanningChainCoordinator
             throw new InvalidOperationException(
                 $"Child {bad.Id} is in status {bad.Status}; expected Manual or Planned.");
 
+        // Worker queue picker requires the "agent" tag — attach it so children are pickable.
+        var agentTag = await ctx.Tags.FirstOrDefaultAsync(t => t.Name == "agent", ct);
         for (int i = 0; i < children.Count; i++)
+        {
             children[i].Status = i == 0 ? TaskStatus.Queued : TaskStatus.Waiting;
+            if (agentTag is not null && !children[i].Tags.Any(t => t.Id == agentTag.Id))
+                children[i].Tags.Add(agentTag);
+        }
 
         await ctx.SaveChangesAsync(ct);
     }