add Go2JoystickFlat task owner and retire G1_walk_flat_amp task

[lemon-star608]

Summary

• 新增 FlashSAC 的 Go2JoystickFlat task owner 配置
• 淘汰已废弃的 flashsac/g1_walk_flat_amp/mujoco owner，并将相关测试切换到仍受支持的 FlashSAC G1 owner。
• 更新后端支持矩阵：加入 FlashSAC Go2 测试覆盖，移除已淘汰的 AMP owner。

Linked Work

• Issue: Work: 稳定 FlashSAC 在 UniLab 中训练 Go2 & G1 并补充算法对比文档 #262
• Milestone:

Validation

• make check
• uv run pytest -m "not slow"
• Additional task-specific validation listed below

Commands actually run:

Impact

• Backend impact: mujoco
• Platform impact: both
• Training effect expected: yes

Artifacts

• tensorboard（红线43s，绿线90s）:

• video / screenshot:

go2_43s.mp4

go2_90s.mp4

Checklist

• Added or updated tests where needed
• Updated docs if behavior or workflow changed
• Linked the driving issue
• Noted any follow-up work explicitly