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,

tests/ClaudeDo.Worker.Tests/External/BatchMcpToolsTests.cs

@@ -623,4 +623,38 @@ public sealed class BatchMcpToolsTests : IDisposable
         Assert.Equal(2, progress.Reports.Count);
         Assert.Equal(2, progress.Reports[1].Total);
     }
+
+    [Fact]
+    public async Task BatchGetTasks_FailedTaskWithFailureReason_ReturnsReasonWhenRequested()
+    {
+        var listId = await SeedListAsync();
+        var task = await SeedTaskAsync(listId, "failed", TaskStatus.Failed);
+        task.FailureReason = "max-turns-exceeded";
+        await _tasks.UpdateAsync(task);
+        var sut = BuildSut();
+
+        var results = await sut.BatchGetTasks(
+            new[] { task.Id }, includeDescription: true, fields: new[] { "failureReason" },
+            cancellationToken: CancellationToken.None);
+
+        var found = results.Single(r => r.Id == task.Id).TaskFull!;
+        Assert.Equal("max-turns-exceeded", found.FailureReason);
+        Assert.Null(found.Description);
+        Assert.Null(found.Status);
+    }
+
+    [Fact]
+    public async Task BatchGetTasks_IdleTaskWithoutFailureReason_ReturnsNullWhenRequested()
+    {
+        var listId = await SeedListAsync();
+        var task = await SeedTaskAsync(listId, "idle", TaskStatus.Idle);
+        var sut = BuildSut();
+
+        var results = await sut.BatchGetTasks(
+            new[] { task.Id }, includeDescription: true, fields: new[] { "failureReason" },
+            cancellationToken: CancellationToken.None);
+
+        var found = results.Single(r => r.Id == task.Id).TaskFull!;
+        Assert.Null(found.FailureReason);
+    }
 }