refactor(claude-do): refactor(worker): move the merge and worktree MCP tools out

`src/ClaudeDo.Worker/External/ExternalMcpService.cs` ist **1897 Zeilen** (nachgezählt 2026-09-02; die Datei ist seit dem Anlegen dieses Tasks von 1696 weiter gewachsen): ~30 DTO-Records oben plus ~25 Tool-Methoden — während **13** Geschwisterdateien `*McpTools.cs` im selben Ordner das Topic-Split-Muster schon tragen (`ListMcpTools`, `ConfigMcpTools`, `BatchMcpTools`, `NotesMcpTools`, `QueueStateMc

ClaudeDo-Task: 1862e160-ffde-4a50-aa03-bb7251f56088

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

@@ -81,7 +81,6 @@ public sealed class AddSubtaskToolTests : IDisposable
         var queue = new QueueService(dbFactory, runner, cfg, NullLogger<QueueService>.Instance, waker, picker, overrideSlot, state, runCancels,
             new FakeUsageGate(), usageState, broadcaster,
             new UsageLimitAutoContinueCoordinator(dbFactory, usageState, overrideSlot, broadcaster, NullLogger<UsageLimitAutoContinueCoordinator>.Instance));
-        var maintenance = new WorktreeMaintenanceService(dbFactory, git, NullLogger<WorktreeMaintenanceService>.Instance);
         var merge = new TaskMergeService(dbFactory, git, broadcaster, state, new VerifyCommandRunner(), NullLogger<TaskMergeService>.Instance);
         var aggregator = new PlanningAggregator(dbFactory, git, NullLogger<PlanningAggregator>.Instance);
         var planningMerge = new PlanningMergeOrchestrator(
@@ -89,7 +88,7 @@ public sealed class AddSubtaskToolTests : IDisposable
         return new ExternalMcpService(
             _tasks, _lists, queue, broadcaster,
             state,
-            git, dbFactory, maintenance, merge, planningMerge,
+            git, dbFactory, merge, planningMerge,
             new BaseDirtyChecker(git, NullLogger<BaseDirtyChecker>.Instance));
     }