fix(worker-client): stop swallowing mutating hub call failures

TryInvokeAsync catches every exception and returns null, which is fine
for read-only calls but hid real HubException reasons behind a generic
"offline" message for the 7 mutating call sites (RestoreDefaultAgents,
UpsertPrimeSchedule, AddDailyNote, CleanupFinishedWorktrees,
ResetAllWorktrees, ForceRemoveWorktree, BuildPlanningIntegrationBranch)
— the same bug class fixed for ApproveReview in e1807fd. Each of the 22
TryInvokeAsync call sites was audited; the 15 read-only ones are left
unchanged (empty/offline is the right display). For the 7 switched to
a direct hub invoke, every caller was checked and, where it had no
catch, one was added so the exception surfaces (StatusMessage,
ShowErrorAsync/CombinedWarning) instead of crashing.

tests/ClaudeDo.Ui.Tests/ViewModels/DiffViewerViewModelTests.cs

@@ -21,11 +21,14 @@ public class DiffViewerViewModelTests
     {
         public IReadOnlyList<SubtaskDiffDto> AggregateResult { get; set; } = Array.Empty<SubtaskDiffDto>();
         public CombinedDiffResultDto? CombinedResult { get; set; }
+        public string? CombinedException { get; set; }
 
         public override Task<IReadOnlyList<SubtaskDiffDto>> GetPlanningAggregateAsync(string planningTaskId) =>
             Task.FromResult(AggregateResult);
         public override Task<CombinedDiffResultDto?> BuildPlanningIntegrationBranchAsync(string planningTaskId, string targetBranch) =>
-            Task.FromResult(CombinedResult);
+            CombinedException is not null
+                ? throw new Exception(CombinedException)
+                : Task.FromResult(CombinedResult);
     }
 
     // ── Files mode: commit-range guards (ported from DiffModal) ──
@@ -193,4 +196,26 @@ public class DiffViewerViewModelTests
         Assert.NotNull(vm.CombinedWarning);
         Assert.NotEmpty(vm.CombinedWarning!);
     }
+
+    [Fact]
+    public async Task Planning_ToggleCombined_WhenWorkerThrows_ShowsExceptionMessage()
+    {
+        var fake = new FakePlanningWorker
+        {
+            AggregateResult = new[] { new SubtaskDiffDto("s1", "First", "b1", "base1", "head1", null, "DIFF-A") },
+            CombinedException = "planning task not found",
+        };
+        var vm = new DiffViewerViewModel(null!, fake);
+        vm.ConfigurePlanning("plan-1", "main");
+        await vm.LoadAsync();
+
+        vm.IsCombinedMode = true;
+
+        var deadline = DateTime.UtcNow.AddSeconds(5);
+        while (DateTime.UtcNow < deadline && vm.IsLoadingCombined) await Task.Delay(10);
+
+        Assert.NotNull(vm.CombinedWarning);
+        Assert.Contains("planning task not found", vm.CombinedWarning);
+        Assert.Equal("", vm.DisplayedDiff);
+    }
 }