test: dedupe the nested test doubles into one shared file

TestDbFactory was redeclared as a private nested class in 40 test files,
NullServiceProvider and StubNotesApi in 14 each, DefaultStub in 5 — 73
declarations, all semantically identical (StubNotesApi differed only in
formatting and type qualification). They now live in TestDoubles.cs next to the
existing StubWorkerClient, which was already the shared-double pattern in this
project.

tests/ClaudeDo.Ui.Tests/ViewModels/WorktreesOverviewReconcileTickTests.cs

@@ -21,11 +21,6 @@ public class WorktreesOverviewReconcileTickTests
             => Task.FromResult<MergeTargetsDto?>(new MergeTargetsDto("main", new[] { "main" }, "chore(list): merge t"));
     }
 
-    private sealed class NoopMergeCoordinator : IMergeCoordinator
-    {
-        public Task ResolveConflictAsync(string taskId, string targetBranch) => Task.CompletedTask;
-    }
-
     private static WorktreeOverviewDto Wt(string taskId, WorktreeState state = WorktreeState.Active) =>
         new(taskId, $"Task {taskId}", TaskStatus.WaitingForReview, "L1", "Work",
             $@"C:\wt\{taskId}", $"claudedo/{taskId}", "base0", state, "+1 -0",
@@ -35,7 +30,8 @@ public class WorktreesOverviewReconcileTickTests
     {
         var worker = new FakeWorker { Worktrees = { Wt("a"), Wt("b"), Wt("c") } };
         var vm = new WorktreesOverviewModalViewModel(
-            worker, () => throw new InvalidOperationException("no diff vm in this test"), new NoopMergeCoordinator());
+            worker, () => throw new InvalidOperationException("no diff vm in this test"),
+            new ClaudeDo.Ui.Services.MergeCoordinator());
         vm.Configure("L1", "Work");
         await vm.LoadAsync();
         return (vm, worker);