feat(data): split list-handler prompt into triage/wait/merge roles

Adds HandlerTriageAlias/HandlerWaitAlias/HandlerMergeAlias to ModelRegistry and
replaces PromptKind.MergeHelperExecute with MergeHelperWait (phase 3 only) and
MergeHelperMerge (phases 4-5), so the list handler can run as three
cost-scoped sessions instead of two. Triage's Phase 2 now checks/sets a wide
verify command via get_list_config/set_list_config once per run. Merge now
merges before reruns, delegates diff review to a sonnet subagent, rejects
0-file diffs, tracks the merged-but-not-Done verify-gate outcome, and caps
reruns at one per task via handoff_list_handler's new nextPhase parameter.

InteractiveLaunchSpecService.cs still references the removed
PromptKind.MergeHelperExecute and fails to build -- wiring the Worker/UI side
onto the new roles is a follow-up task.

src/ClaudeDo.Data/Models/ModelRegistry.cs

@@ -9,6 +9,9 @@ public static class ModelRegistry
 
     public const string DefaultAlias = "sonnet";
     public const string PlanningAlias = "opus";
+    public const string HandlerTriageAlias = "opus";
+    public const string HandlerWaitAlias = "sonnet";
+    public const string HandlerMergeAlias = "opus";
 
     public const string ListDefaultSentinel = "(default)";
     public const string TaskInheritSentinel = "(inherit)";