feat(worker): add wait_for_task_change MCP tool

Replaces the list handler's Start-Sleep + blind get_task poll (Phase 3
of the merge-helper prompt) with a blocking MCP tool that returns as
soon as a task leaves Queued/Running, or times out. Implemented as an
async DB poll (short-lived DbContext, 500ms delay, no held connection)
rather than hooking HubBroadcaster, keeping the existing broadcast
callers untouched. timeoutSeconds is clamped server-side to 170s, under
the list handler's 200s MCP_TOOL_TIMEOUT.

src/ClaudeDo.Worker/Program.cs

@@ -294,6 +294,7 @@ if (cfg.ExternalMcpPort > 0)
     externalBuilder.Services.AddScoped<AgentMcpTools>();
     externalBuilder.Services.AddScoped<LifecycleMcpTools>();
     externalBuilder.Services.AddScoped<AppSettingsMcpTools>();
+    externalBuilder.Services.AddScoped<TaskWaitMcpTools>();
     externalBuilder.Services.AddSingleton(app.Services.GetRequiredService<AttachmentStore>());
     externalBuilder.Services.AddScoped<TaskAttachmentRepository>();
     externalBuilder.Services.AddScoped<AttachmentMcpTools>();
@@ -308,6 +309,7 @@ if (cfg.ExternalMcpPort > 0)
         .WithTools<AgentMcpTools>()
         .WithTools<LifecycleMcpTools>()
         .WithTools<AppSettingsMcpTools>()
+        .WithTools<TaskWaitMcpTools>()
         .WithTools<AttachmentMcpTools>();
     externalBuilder.WebHost.UseUrls($"http://127.0.0.1:{cfg.ExternalMcpPort}");