fix(worker): thread handler task id into list-handler kickoff prompts

Neither the initial nor the handoff kickoff ever told a list-handler
session its own handler task id, so handoff_list_handler(taskId, ...)
was unrenderable -- the handoff chain broke exactly where it was needed
(#200/#201 on 2026-08-21).

Add {handlerTaskId} to both MergeHelperInitialDefault and
MergeHelperHandoffDefault, thread a handlerTaskId parameter through
BuildForMergeHelperAsync (interface, WorkerHub.GetMergeHelperLaunchSpec,
IWorkerClient/WorkerClient, and the MissionControlViewModel call site,
which already had the id from CreateMergeHelperTaskAsync but never
passed it on), and render it in BuildForMergeHelperHandoffAsync from the
taskId parameter it already receives.

RenderTemplate leaves unknown/missing tokens untouched, so a
user-edited override without the new token still renders fine -- no
forced migration for override users.

src/ClaudeDo.Ui/Services/Interfaces/IWorkerClient.cs

@@ -130,8 +130,10 @@ public interface IWorkerClient : INotifyPropertyChanged
     /// no task, no worktree.</summary>
     Task<LaunchSpec> GetAdHocLaunchSpecAsync(string directory, CancellationToken ct = default);
     /// <summary>Launch spec for an embedded ConPTY "merge helper" session that drives the given
-    /// tasks to a merged/Done state. listId scopes the session (and cwd) to that list.</summary>
-    Task<LaunchSpec> GetMergeHelperLaunchSpecAsync(IReadOnlyList<string> taskIds, string listId, CancellationToken ct = default);
+    /// tasks to a merged/Done state. listId scopes the session (and cwd) to that list.
+    /// handlerTaskId (from CreateMergeHelperTaskAsync) is rendered into the brief so the session
+    /// can call handoff_list_handler on its own handler task.</summary>
+    Task<LaunchSpec> GetMergeHelperLaunchSpecAsync(IReadOnlyList<string> taskIds, string listId, string handlerTaskId, CancellationToken ct = default);
     /// <summary>Creates the ClaudeDo task that owns a list-handler run (one per run, Idle/IsManual,
     /// never queued) so the ConPTY tile can be task-based instead of ad-hoc. Returns the new task id.</summary>
     Task<string> CreateMergeHelperTaskAsync(

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -614,8 +614,8 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
     public async Task<LaunchSpec> GetAdHocLaunchSpecAsync(string directory, CancellationToken ct = default)
         => await InvokeTimedAsync<LaunchSpec>("GetAdHocLaunchSpec", () => _hub.InvokeAsync<LaunchSpec>("GetAdHocLaunchSpec", directory, ct));
 
-    public async Task<LaunchSpec> GetMergeHelperLaunchSpecAsync(IReadOnlyList<string> taskIds, string listId, CancellationToken ct = default)
-        => await InvokeTimedAsync<LaunchSpec>("GetMergeHelperLaunchSpec", () => _hub.InvokeAsync<LaunchSpec>("GetMergeHelperLaunchSpec", taskIds, listId, ct));
+    public async Task<LaunchSpec> GetMergeHelperLaunchSpecAsync(IReadOnlyList<string> taskIds, string listId, string handlerTaskId, CancellationToken ct = default)
+        => await InvokeTimedAsync<LaunchSpec>("GetMergeHelperLaunchSpec", () => _hub.InvokeAsync<LaunchSpec>("GetMergeHelperLaunchSpec", taskIds, listId, handlerTaskId, ct));
 
     public async Task<string> CreateMergeHelperTaskAsync(
         IReadOnlyList<string> taskIds, string listId, string title, string descriptionHeader, CancellationToken ct = default)