feat(list-handler): hand off to a fresh session after Phase 2

The merge-helper ("Let Claude handle it") system prompt now calls a new
handoff_list_handler MCP tool once every surviving task is enhanced,
instead of continuing into Phases 3-5 in the same session -- avoiding
paying for Phases 0-2's dedupe/rewrite context on every polling round of
the run/review/merge phases.

The tool broadcasts HandoffRequested; Mission Control opens a second
ConPTY tile for the SAME handler task id (no new task, HandlerBaseCommit
untouched) running a fresh handoff brief that starts at Phase 3. The
original tile stays open. Adds PromptKind.MergeHelperHandoff,
InteractiveLaunchSpecService.BuildForMergeHelperHandoffAsync, and the
GetMergeHelperHandoffLaunchSpec hub method.

src/ClaudeDo.Worker/Program.cs

@@ -300,6 +300,7 @@ if (cfg.ExternalMcpPort > 0)
     externalBuilder.Services.AddScoped<RunHistoryMcpTools>();
     externalBuilder.Services.AddScoped<AgentMcpTools>();
     externalBuilder.Services.AddScoped<LifecycleMcpTools>();
+    externalBuilder.Services.AddScoped<HandoffMcpTools>();
     externalBuilder.Services.AddScoped<AppSettingsMcpTools>();
     externalBuilder.Services.AddScoped<TaskWaitMcpTools>();
     externalBuilder.Services.AddSingleton(app.Services.GetRequiredService<AttachmentStore>());
@@ -315,6 +316,7 @@ if (cfg.ExternalMcpPort > 0)
         .WithTools<RunHistoryMcpTools>()
         .WithTools<AgentMcpTools>()
         .WithTools<LifecycleMcpTools>()
+        .WithTools<HandoffMcpTools>()
         .WithTools<AppSettingsMcpTools>()
         .WithTools<TaskWaitMcpTools>()
         .WithTools<AttachmentMcpTools>();