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.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 --