feat(worker): wire list-handler phase parameter through handoff chain

Threads a nextPhase parameter (wait/merge/wait_final/merge_final, validated
by the new MergeHelperPhase) from handoff_list_handler through
HubBroadcaster/WorkerHub into InteractiveLaunchSpecService, which now picks
the next session's system prompt (MergeHelperWait/MergeHelperMerge) and
model (HandlerWaitAlias/HandlerMergeAlias) from it instead of hardcoding the
old two-phase Execute prompt -- this also fixes a build break left by the
prior prompt-split task, which removed PromptKind.MergeHelperExecute without
updating its only caller.

Also sets --model/--effort/--permission-mode explicitly for every list-handler
session (Triage included) via PermissionModeResolver instead of inheriting the
CLI's ambient model and hardcoding "auto", and adds Task to the merge-helper
allowlist so the Merge phase can delegate diff reviews to subagents.

src/ClaudeDo.Worker/Hub/HubBroadcaster.cs

@@ -34,8 +34,8 @@ public sealed class HubBroadcaster : IPrimeBroadcaster, IRefineBroadcaster
 
     // A running list-handler session called handoff_list_handler at the end of Phase 2 -- the UI
     // opens a second ConPTY tile for the same handler task id to carry out Phases 3-5.
-    public Task HandoffRequested(string taskId, IReadOnlyList<string> survivingTaskIds) =>
-        _hub.Clients.All.SendAsync("HandoffRequested", taskId, survivingTaskIds);
+    public Task HandoffRequested(string taskId, IReadOnlyList<string> survivingTaskIds, string nextPhase) =>
+        _hub.Clients.All.SendAsync("HandoffRequested", taskId, survivingTaskIds, nextPhase);
 
     public Task ListUpdated(string listId) =>
         _hub.Clients.All.SendAsync("ListUpdated", listId);