feat: add Go1JoystickFlat2 env and SAC configs

[ShineSea]

Summary

• Added Go1JoystickFlat2 as a Go1 flat locomotion environment variant.
• Registered the environment for both MuJoCo and Motrix backends.
• Added SAC owner configs for go1_joystick_flat on MuJoCo and Motrix.

This enables SAC training through the standard CLI route:

uv run train --algo sac --task go1_joystick_flat --sim mujoco
uv run train --algo sac --task go1_joystick_flat --sim motrix

[TATP-233]

名称不好，令人疑惑，建议在原任务文件中修改，通过配置文件的 scale 来调整具体行为（action reward）