refactor(worker): extract ProgressReporter from TaskMergeService

Pulls the MCP idle-timeout progress loop out of TaskMergeService into a
standalone ProgressReporter (Lifecycle namespace) shared by the verify
gate and preview-verify call sites, and adds a per-item i/n overload
(ReportItem) for upcoming batch progress in D2/D3.

tests/ClaudeDo.Worker.Tests/Lifecycle/ProgressReporterTests.cs

@@ -0,0 +1,58 @@
+using ClaudeDo.Worker.Lifecycle;
+using ModelContextProtocol;
+using Xunit;
+
+namespace ClaudeDo.Worker.Tests.Lifecycle;
+
+// A synchronously-collecting IProgress<T>: Progress<T> marshals through the SynchronizationContext
+// captured at construction, which is unreliable to assert on immediately in a test.
+file sealed class SyncProgress<T> : IProgress<T>
+{
+    public readonly List<T> Reports = new();
+    public void Report(T value) => Reports.Add(value);
+}
+
+public sealed class ProgressReporterTests
+{
+    [Fact]
+    public async Task RunAsync_NoProgressOrOnTick_ReturnsWorkResultWithoutTicking()
+    {
+        var result = await ProgressReporter.RunAsync(Task.FromResult(42), TimeSpan.FromMilliseconds(10), null, "message");
+
+        Assert.Equal(42, result);
+    }
+
+    [Fact]
+    public async Task RunAsync_SlowWork_ReportsProgressOnEachTick()
+    {
+        var progress = new SyncProgress<ProgressNotificationValue>();
+        var work = Task.Delay(250).ContinueWith(_ => "done");
+
+        var result = await ProgressReporter.RunAsync(work, TimeSpan.FromMilliseconds(50), progress, "waiting");
+
+        Assert.Equal("done", result);
+        Assert.NotEmpty(progress.Reports);
+        Assert.All(progress.Reports, r => Assert.Contains("waiting", r.Message));
+    }
+
+    [Fact]
+    public void ReportItem_ReportsMessageAndProgressTotalForEachItem()
+    {
+        var progress = new SyncProgress<ProgressNotificationValue>();
+
+        for (var i = 1; i <= 3; i++)
+            ProgressReporter.ReportItem(progress, "cleaning worktrees", i, 3);
+
+        Assert.Equal(3, progress.Reports.Count);
+        Assert.Equal("cleaning worktrees (1/3)", progress.Reports[0].Message);
+        Assert.Equal("cleaning worktrees (2/3)", progress.Reports[1].Message);
+        Assert.Equal("cleaning worktrees (3/3)", progress.Reports[2].Message);
+        Assert.Equal(3, progress.Reports[2].Total);
+    }
+
+    [Fact]
+    public void ReportItem_NoProgress_DoesNotThrow()
+    {
+        ProgressReporter.ReportItem(null, "cleaning worktrees", 1, 3);
+    }
+}