refactor: drop the remaining single-implementation interfaces

IBaseDirtyChecker, IInteractiveLaunchSpecService and the LaunchSpec wrapper
had one implementation and one caller each; ProcessRunnerAdapter existed only
to give a static class an interface, and InstallArtifactLocator used
inheritance for two constructor arguments. The external MCP container now
shares its singletons through a Share<T> helper instead of 17 near-identical
registrations.

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -202,7 +202,7 @@ public sealed class ExternalMcpService
     private readonly WorktreeMaintenanceService _maintenance;
     private readonly TaskMergeService _merge;
     private readonly PlanningMergeOrchestrator _planningMerge;
-    private readonly IBaseDirtyChecker _baseDirtyChecker;
+    private readonly BaseDirtyChecker _baseDirtyChecker;
 
     public ExternalMcpService(
         TaskRepository tasks,
@@ -215,7 +215,7 @@ public sealed class ExternalMcpService
         WorktreeMaintenanceService maintenance,
         TaskMergeService merge,
         PlanningMergeOrchestrator planningMerge,
-        IBaseDirtyChecker baseDirtyChecker)
+        BaseDirtyChecker baseDirtyChecker)
     {
         _tasks = tasks;
         _lists = lists;