docs(action) clarify AgiBotWorld URDF scope

[sylvesterkaczmarek]

Summary

• clarify that the public AgiBotWorld FK path uses G1_omnipicker_calibrated.urdf as a kinematics reference
• document that the URDF's referenced visual mesh package is not bundled in robot_assets
• explain that the FK loader strips visual and collision elements before constructing its MuJoCo kinematics model
• clarify that the committed file is not a complete renderable description for every AgiBotWorld gripper variant

Why

Issue #305 asks why the AgiBotWorld Beta data appears to use a CTEK gripper while the available URDF is an omnipicker description, and asks for the complete mesh-backed URDF.

The current public Cosmos Framework hard-codes G1_omnipicker_calibrated.urdf for the AgiBotWorld FK path. The URDF contains package://genie_robot_description/meshes/... references, but the corresponding mesh package is not included in the public robot_assets directory. This does not block the current action FK path because the loader explicitly removes <visual> and <collision> elements before loading the model into MuJoCo.

This documentation change makes the scope of the published robot description explicit and avoids implying that it is a complete renderable CTEK/omnipicker asset package.

Validation

Documentation-only change. Cross-checked against agibot_spec.py, agibot_fk.py, the committed G1_omnipicker_calibrated.urdf, and the current robot_assets directory in NVIDIA/cosmos-framework.

Addresses #305