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.
mika kuns
9c4a63d2c5