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.Data/PromptFiles.cs

@@ -547,6 +547,9 @@ public static class PromptFiles
         Scope: {scope}
         Repo: {repo}
 
+        Your own handler task id (for handoff_list_handler / submit_task_for_review — do NOT confuse
+        this with the ids of the tasks you are handling below): {handlerTaskId}
+
         Handle the following tasks. Work Phases 0–5 as your instructions describe, asking me whenever you are unsure.
 
         {tasks}
@@ -560,6 +563,9 @@ public static class PromptFiles
         Scope: {scope}
         Repo: {repo}
 
+        Your own handler task id (for handoff_list_handler / submit_task_for_review — do NOT confuse
+        this with the ids of the surviving tasks below): {handlerTaskId}
+
         A prior session already read, deduped and enhanced this list's tasks. Pick up at phase 3
         for the tasks below — their descriptions are already sharpened.
 

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)

src/ClaudeDo.Ui/ViewModels/MissionControlViewModel.cs

@@ -279,7 +279,7 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
             }
 
             AddConPtyPane(new ConPtyPaneViewModel(taskId, title,
-                () => DescribeAsync(() => _worker.GetMergeHelperLaunchSpecAsync(taskIds, listId))));
+                () => DescribeAsync(() => _worker.GetMergeHelperLaunchSpecAsync(taskIds, listId, taskId))));
         }
         finally
         {

src/ClaudeDo.Worker/Hub/WorkerHub.cs

@@ -864,12 +864,13 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
 
     // Builds the launch spec for an embedded ConPTY "merge helper" session that drives the given
     // tasks to a merged/Done state via the mcp__claudedo__* tools. listId scopes the brief label
-    // and cwd to that list.
-    public Task<LaunchSpec> GetMergeHelperLaunchSpec(string[] taskIds, string listId) => HubGuard(() =>
+    // and cwd to that list. handlerTaskId is the id returned by CreateMergeHelperTask -- it is
+    // rendered into the brief so the session can call handoff_list_handler on itself.
+    public Task<LaunchSpec> GetMergeHelperLaunchSpec(string[] taskIds, string listId, string handlerTaskId) => HubGuard(() =>
     {
         if (_interactiveLaunchSpec is null)
             throw new InvalidOperationException("Interactive launch spec service is not configured.");
-        return _interactiveLaunchSpec.BuildForMergeHelperAsync(taskIds, listId, Context.ConnectionAborted);
+        return _interactiveLaunchSpec.BuildForMergeHelperAsync(taskIds, listId, handlerTaskId, Context.ConnectionAborted);
     });
 
     // Creates the ClaudeDo task that owns a list-handler run, before the ConPTY tile opens --

src/ClaudeDo.Worker/Runner/InteractiveLaunchSpecService.cs

@@ -241,7 +241,7 @@ public sealed class InteractiveLaunchSpecService : IInteractiveLaunchSpecService
     private const string MergeHelperAllowedTools =
         "mcp__claudedo__*,Read,Grep,Glob,Edit,Bash,WebFetch,WebSearch,Skill,Task";
 
-    public async Task<LaunchSpec> BuildForMergeHelperAsync(IReadOnlyList<string> taskIds, string listId, CancellationToken ct)
+    public async Task<LaunchSpec> BuildForMergeHelperAsync(IReadOnlyList<string> taskIds, string listId, string handlerTaskId, CancellationToken ct)
     {
         if (taskIds.Count == 0)
             throw new InvalidOperationException("No tasks selected for the list handler.");
@@ -278,6 +278,7 @@ public sealed class InteractiveLaunchSpecService : IInteractiveLaunchSpecService
             {
                 ["scope"] = $"List: {list.Name}",
                 ["repo"] = repoDir,
+                ["handlerTaskId"] = handlerTaskId,
                 ["tasks"] = string.Join("\n", briefLines),
             }), ct);
 
@@ -371,6 +372,7 @@ public sealed class InteractiveLaunchSpecService : IInteractiveLaunchSpecService
             {
                 ["scope"] = $"List: {list.Name}",
                 ["repo"] = repoDir,
+                ["handlerTaskId"] = taskId,
                 ["tasks"] = string.Join("\n", briefLines),
                 ["finalNote"] = finalNote,
             }), ct);

src/ClaudeDo.Worker/Runner/Interfaces/IInteractiveLaunchSpecService.cs

@@ -34,9 +34,11 @@ public interface IInteractiveLaunchSpecService
     /// given tasks to a merged/Done state via the mcp__claudedo__* tools. Writes a per-session
     /// system prompt + task brief under ~/.todo-app/merge-helper-sessions/<guid> and exposes
     /// that dir plus the list's repo dir via --add-dir. cwd is the list's working directory.
+    /// 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>
-    Task<LaunchSpec> BuildForMergeHelperAsync(IReadOnlyList<string> taskIds, string listId, CancellationToken ct);
+    Task<LaunchSpec> BuildForMergeHelperAsync(IReadOnlyList<string> taskIds, string listId, string handlerTaskId, CancellationToken ct);
 
     /// <summary>Creates the ClaudeDo task that hosts a list-handler run (Mission Control's
     /// "Let Claude handle it") and stamps the list repo's current HEAD as the review range's