feat(worker): fast-forward finished children into the parent's unit branch

Planning parents get their own worktree on Queue plan, children fork from the
unit head and are absorbed via merge --ff-only before Done (ff failure -> Failed,
interactive child approve -> absorb instead of merge to the target).

tests/ClaudeDo.Worker.Tests/Infrastructure/TaskStateServiceBuilder.cs

@@ -26,7 +26,8 @@ public static class TaskStateServiceBuilder
     public static Built Build(
         IDbContextFactory<ClaudeDoDbContext> dbFactory,
         Func<IActiveMergeState>? mergeState = null,
-        BaseDirtyChecker? baseDirtyChecker = null)
+        BaseDirtyChecker? baseDirtyChecker = null,
+        ClaudeDo.Worker.Runner.UnitBranchService? unitBranch = null)
     {
         var hub = new CapturingHubContext();
         var broadcaster = new HubBroadcaster(hub);
@@ -46,7 +47,7 @@ public static class TaskStateServiceBuilder
             NullLogger<TicketStatusSync>.Instance);
 
         TaskStateService? state = null;
-        var chain = new PlanningChainCoordinator(dbFactory, () => state!);
+        var chain = new PlanningChainCoordinator(dbFactory, () => state!, unitBranch);
         state = new TaskStateService(
             dbFactory,
             broadcaster,