feat(worker): add post-merge verification gate for list merges

Per-list optional VerifyCommand (list_config.verify_command) runs via
VerifyCommandRunner in the list's working dir right after a successful
merge/continue-merge, before the task is allowed to reach Done. A
non-zero exit or timeout leaves the merge in place but keeps the task
out of Done and reports StatusVerifyFailed with an output excerpt
through MergeResultDto/review_task; no command configured behaves
exactly as before. Merges against the same repo are now serialized
per working dir so a running verify can't be interrupted by a second
merge landing mid-build. Adds the field to the List Settings modal
(en/de localized) and covers success/failure/timeout in
TaskMergeServiceTests + VerifyCommandRunnerTests.

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

@@ -83,6 +83,20 @@
           </Border>
         </StackPanel>
 
+        <!-- VERIFICATION -->
+        <StackPanel Spacing="0">
+          <TextBlock Classes="section-label" Text="{loc:Tr modals.listSettings.sectionVerify}"/>
+          <Border Classes="section">
+            <StackPanel Spacing="4">
+              <TextBlock Classes="field-label" Text="{loc:Tr modals.listSettings.verifyCommand}"/>
+              <TextBox Text="{Binding VerifyCommand, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
+                       PlaceholderText="{loc:Tr modals.listSettings.verifyCommandPlaceholder}"/>
+              <TextBlock Text="{loc:Tr modals.listSettings.verifyCommandHint}"
+                         Opacity="0.6" FontSize="12" TextWrapping="Wrap"/>
+            </StackPanel>
+          </Border>
+        </StackPanel>
+
       </StackPanel>
     </ScrollViewer>