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>

src/ClaudeDo.Worker/Queue/QueueSlotState.cs

@@ -0,0 +1,8 @@
+namespace ClaudeDo.Worker.Queue;
+
+public sealed class QueueSlotState
+{
+    public required string TaskId { get; init; }
+    public required DateTime StartedAt { get; init; }
+    public required CancellationTokenSource Cts { get; init; }
+}