refactor(worker): extract OverrideSlotService and reorganize Worker/Services into domain folders

Slice 5 of the worker state consolidation refactor.

OverrideSlotService (new in Worker/Queue/) owns RunNow, ContinueTask,
and the override-slot piece of CancelTask. QueueService keeps the
queue-slot guard for "task is already running" rejection and delegates
to OverrideSlotService for execution; CancelTask tries the override
slot first, then the queue slot. QueueSlotState is extracted to its own
file.

Folder reorg (via git mv to preserve history):
- Worker/Queue/      QueueService, OverrideSlotService, QueueSlotState
                     (alongside existing waker/picker)
- Worker/Lifecycle/  StaleTaskRecovery, TaskResetService, TaskMergeService
- Worker/Worktrees/  WorktreeMaintenanceService
- Worker/Agents/     AgentFileService, DefaultAgentSeeder

Worker/Services/ folder removed. All consumers updated to the new
namespaces (Program.cs, WorkerHub, ExternalMcpService,
PlanningMergeOrchestrator, all Worker tests).

OverrideSlotService is registered as a DI singleton in both the main
worker app and the external MCP app.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -5,8 +5,8 @@ using ClaudeDo.Data.Repositories;
 using ClaudeDo.Worker.Config;
 using ClaudeDo.Worker.External;
 using ClaudeDo.Worker.Hub;
+using ClaudeDo.Worker.Queue;
 using ClaudeDo.Worker.Runner;
-using ClaudeDo.Worker.Services;
 using ClaudeDo.Worker.Tests.Infrastructure;
 using ClaudeDo.Worker.Tests.Services;
 using Microsoft.AspNetCore.SignalR;
@@ -117,7 +117,8 @@ public sealed class ExternalMcpServiceTests : IDisposable
             NullLogger<TaskRunner>.Instance, TaskStateServiceBuilder.Build(dbFactory).State);
         var waker = new ClaudeDo.Worker.Queue.QueueWaker();
         var picker = new ClaudeDo.Worker.Queue.QueuePicker(dbFactory);
-        return new QueueService(dbFactory, runner, cfg, NullLogger<QueueService>.Instance, waker, picker);
+        var overrideSlot = new OverrideSlotService(dbFactory, runner, NullLogger<OverrideSlotService>.Instance);
+        return new QueueService(dbFactory, runner, cfg, NullLogger<QueueService>.Instance, waker, picker, overrideSlot);
     }
 
     [Fact]