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/WorkerHub.cs

@@ -855,11 +855,11 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
     // Builds the launch spec for the fresh ConPTY session a merge-helper run hands off to once
     // Phase 2 (enhance) is done -- reuses the SAME handler task id (no new task created), only a
     // new session dir + handoff kickoff naming the surviving tasks.
-    public Task<LaunchSpec> GetMergeHelperHandoffLaunchSpec(string taskId, string[] survivingTaskIds) => HubGuard(() =>
+    public Task<LaunchSpec> GetMergeHelperHandoffLaunchSpec(string taskId, string[] survivingTaskIds, string nextPhase) => HubGuard(() =>
     {
         if (_interactiveLaunchSpec is null)
             throw new InvalidOperationException("Interactive launch spec service is not configured.");
-        return _interactiveLaunchSpec.BuildForMergeHelperHandoffAsync(taskId, survivingTaskIds, Context.ConnectionAborted);
+        return _interactiveLaunchSpec.BuildForMergeHelperHandoffAsync(taskId, survivingTaskIds, nextPhase, Context.ConnectionAborted);
     });
 
     // Starts a planning session (worktree + prompt files + token, task -> Planning) and returns