Let task agents create and manage subtasks

internal/agents/guidance.go

@@ -75,6 +75,13 @@ func (m *Manager) taskGuidance(ctx context.Context, task api.Task) (string, erro
 		g += fmt.Sprintf("\n- This task was started from the item %q in %s at the repo root. When the work is done, remove that item from %s in this worktree, so the removal lands in the same merge as the work.",
 			*task.TodoItem, todo.File, todo.File)
 	}
+	if task.ParentTaskId != nil {
+		parent, err := m.store.GetTask(ctx, *task.ParentTaskId)
+		if err != nil {
+			return "", err
+		}
+		g += subtaskGuidance(parent)
+	}
 	if task.ForkedFrom == nil {
 		return g, nil
 	}
@@ -85,6 +92,12 @@ func (m *Manager) taskGuidance(ctx context.Context, task api.Task) (string, erro
 	return g + forkGuidance(task, src), nil
 }
 
+func subtaskGuidance(parent api.Task) string {
+	return fmt.Sprintf("\n- This task is a subtask of byre task %q (%s), whose agent created it and manages it. Your final message at the end of each turn is sent to that agent, which may answer with more instructions. "+
+		"Your work reaches %s's branch or the default branch when the subtask is merged.",
+		parent.Title, parent.Id, parent.Id)
+}
+
 func forkGuidance(task, src api.Task) string {
 	return fmt.Sprintf("\n- This conversation was forked from byre task %q at its snapshot %d, into a task of its own: branch %s, worktree %s, which held that snapshot's files when the fork was made. "+
 		"Everything before the fork happened in %s, which belongs to the other task: from now on read and change files only in %s, never in the other worktree, and use paths under it.",