feat(worker): surface the handoff round budget in the merge-helper handoff brief

A handler session only learned the chain was capped when handoff_list_handler
threw at the end of its work, stranding open tasks. The round count already
lived in HandoffRoundTracker the whole time -- it just never reached the
brief. InteractiveLaunchSpecService now reads HandoffRoundTracker.GetState
(read-only, doesn't advance the round) and renders round N of MaxHandoffs
plus the remaining count into the handoff brief, replacing the old one-line
finalNote. The rejection error also names the next action (start a new
handler run) instead of just failing.

tests/ClaudeDo.Worker.Tests/Hub/MergeHelperTaskHubTests.cs

@@ -64,7 +64,7 @@ public sealed class MergeHelperTaskHubTests : IDisposable
         var wtManager = new WorktreeManager(git, factory, new WorkerConfig(), NullLogger<WorktreeManager>.Instance);
         var interactiveLaunchSpec = new InteractiveLaunchSpecService(
             factory, new FakeSessionSkillSeeder(), new FakeSessionSkillRegistry(), wtManager, git,
-            new WorkerConfig { ClaudeBin = "claude" });
+            new WorkerConfig { ClaudeBin = "claude" }, new ClaudeDo.Worker.External.HandoffRoundTracker());
         var built = TaskStateServiceBuilder.Build(factory);
         var reviewSubmission = new InteractiveReviewSubmissionService(factory, git, built.State, wtManager);