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.