feat(ui): triage model picker (opus/fable) in the merge helper start modal

src/ClaudeDo.Worker/Runner/InteractiveLaunchSpecService.cs

@@ -290,8 +290,10 @@ public sealed class InteractiveLaunchSpecService
     /// handlerTaskId (the id returned by CreateMergeHelperTaskAsync) is rendered into the brief so
     /// the session can call handoff_list_handler/submit_task_for_review on its own handler task.
     /// Throws KeyNotFoundException if the list doesn't exist; InvalidOperationException if
-    /// taskIds is empty or the list has no existing working directory.</summary>
-    public async Task<LaunchSpec> BuildForMergeHelperAsync(IReadOnlyList<string> taskIds, string listId, string handlerTaskId, CancellationToken ct)
+    /// taskIds is empty or the list has no existing working directory.
+    /// model overrides the triage session's model (start-modal dropdown); null/unknown falls
+    /// back to <see cref="ModelRegistry.HandlerTriageAlias"/>. Handoff phases are unaffected.</summary>
+    public async Task<LaunchSpec> BuildForMergeHelperAsync(IReadOnlyList<string> taskIds, string listId, string handlerTaskId, string? model, CancellationToken ct)
     {
         if (taskIds.Count == 0)
             throw new InvalidOperationException("No tasks selected for the list handler.");
@@ -344,12 +346,13 @@ public sealed class InteractiveLaunchSpecService
         // longer influences a handler session at all, it would otherwise apply the LIST's effort
         // preset to a completely different model.
         var settings = await new AppSettingsRepository(ctx).GetAsync(ct);
+        var triageModel = ModelRegistry.TryNormalizeAlias(model) ?? ModelRegistry.HandlerTriageAlias;
 
         var args = new List<string>
         {
-            "--model", ModelRegistry.HandlerTriageAlias,
-            "--effort", EffortFor(settings, ModelRegistry.HandlerTriageAlias),
-            "--permission-mode", PermissionModeResolver.Resolve(ModelRegistry.HandlerTriageAlias, "auto"),
+            "--model", triageModel,
+            "--effort", EffortFor(settings, triageModel),
+            "--permission-mode", PermissionModeResolver.Resolve(triageModel, "auto"),
             "--allowedTools", MergeHelperAllowedTools,
             "--add-dir", sessionDir, repoDir,
             "--append-system-prompt-file", systemPromptPath,