#2632 feat: add task-level ATIF trajectory example task and docs

Adds hello-load-atif-trajectory-task-level, which bakes an ATIF trajectory into the image and declares it via
[environment].atif_trajectory_path, so no --load-trajectory flag is needed. Renames hello-load-atif-trajectory to
hello-load-atif-trajectory-run-level to make the run-level/task-level pair explicit, and updates the loading docs with the task-level section, the new field reference, and links to all three example tasks.

---