fix(merge): conventional merge-commit default and live verify progress

The merge commit message was hand-rolled per caller ("Merge task: <title>",
"Merge <branch>", "Merge subtask") and ignored the task's commit type. Every
caller now passes a blank message and TaskMergeService fills in
CommitMessageBuilder.BuildMerge -> {commitType}(list-slug): merge <title> plus the
ClaudeDo-Task trailer; the merge modal prefills it from GetMergeTargets.

A merge whose list has a verify command holds the MergeTask call for minutes (5m46s
on this repo), during which the modal only disabled its button - no spinner, no
message, so a landed merge looked like a dead app. TaskMergeService now broadcasts
MergeProgress(taskId, phase, elapsedSeconds) for the merging and verifying phases
(re-reported every 30s) plus a WorkerLog line when verify starts; the modal shows a
spinner and the localized phase.

src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml

@@ -22,6 +22,15 @@
     <ctl:ModalShell.Footer>
       <StackPanel Orientation="Horizontal" Spacing="8"
                   HorizontalAlignment="Right" VerticalAlignment="Center">
+        <!-- A merge into a list with a verify command holds this dialog for minutes; without a
+             live phase here the disabled button is the only feedback and reads as a dead app. -->
+        <StackPanel Orientation="Horizontal" Spacing="8" VerticalAlignment="Center"
+                    IsVisible="{Binding IsBusy}">
+          <Ellipse Classes="spinner" VerticalAlignment="Center"/>
+          <TextBlock Classes="meta" VerticalAlignment="Center"
+                     Text="{Binding ProgressMessage}"
+                     IsVisible="{Binding ProgressMessage, Converter={x:Static ObjectConverters.IsNotNull}}"/>
+        </StackPanel>
         <Button Classes="btn" Content="{loc:Tr modals.merge.cancel}" Command="{Binding CancelCommand}" MinWidth="90"/>
         <Button Content="{loc:Tr modals.merge.merge}" Classes="primary"
                 Command="{Binding SubmitCommand}"