fix[hatch-pet]: clarify hatch-pet waving loop guidance

[shhac]

Fixes #469

Summary

Clarifies the hatch-pet waving animation guidance so generated rows:

• keep the waving side consistent across frames
• avoid fully idle poses as the first or last frame
• start at the first wave pose
• end on a distinct return pose that loops back into the start

The same guidance is added to the generated row prompt template in prepare_pet_run.py.

Testing

Tested once manually by generating a wave animation with this guidance, and the result looked correct.