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).

src/ClaudeDo.Worker/Program.cs

@@ -112,6 +112,7 @@ builder.Services.AddSingleton<HubBroadcaster>();
 builder.Services.AddSingleton<ConnectedClientTracker>();
 builder.Services.AddSingleton<GitService>();
 builder.Services.AddSingleton<WorktreeManager>();
+builder.Services.AddSingleton<UnitBranchService>();
 builder.Services.AddSingleton<ClaudeArgsBuilder>();
 builder.Services.AddSingleton<TaskRunTokenRegistry>();
 builder.Services.AddSingleton<PendingQuestionRegistry>();