fix(worker): add failureReason field to batch_get_tasks

The tool description claimed Failed tasks carry failureReason, but the field
was missing from BatchTaskDetailDto and rejected when requested in `fields`.
Add failureReason to the DTO, KnownDetailFields, and BuildDetail so list
handlers and other callers can read failure reasons without per-task get_task
calls. Tested: 31 BatchMcpToolsTests pass, including two new cases for
failureReason retrieval on Failed and Idle tasks.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/ClaudeDo.Worker/External/BatchMcpTools.cs

@@ -45,6 +45,7 @@ public sealed record BatchTaskDetailDto(
     int? SortOrder = null,
     int? RoadblockCount = null,
     string? RoadblockText = null,
+    string? FailureReason = null,
     bool DescriptionTruncated = false,
     int? DescriptionFullLength = null,
     bool ResultTruncated = false,
@@ -80,6 +81,7 @@ public sealed class BatchMcpTools
     {
         "listId", "title", "description", "status", "result", "createdBy", "createdAt",
         "startedAt", "finishedAt", "isMyDay", "sortOrder", "roadblockCount", "roadblockText",
+        "failureReason",
     };
 
     private readonly ExternalMcpService _svc;
@@ -187,6 +189,7 @@ public sealed class BatchMcpTools
             SortOrder: Want("sortOrder") ? t.SortOrder : null,
             RoadblockCount: Want("roadblockCount") ? t.RoadblockCount : null,
             RoadblockText: Want("roadblockText") ? ExtractRoadblockText(t.Result) : null,
+            FailureReason: Want("failureReason") ? t.FailureReason : null,
             DescriptionTruncated: descTruncated,
             DescriptionFullLength: descFullLength,
             ResultTruncated: resultTruncated,