Add Onerobot A1 arm

If you would like to see a new model added to the Menagerie collection, please provide us with the following information _before_ sending a pull request. (Ideally, before you start working on converting or tuning the model for MuJoCo.)

**URL for the existing model source (e.g. URDF)**
https://github.com/katazen/onerobot_h1/tree/main/source/h1_reach/h1_reach/assets/mjcf/A1_dual

**License terms of the upstream model**
BSD-3-Clause

**What do you think is required to convert and tune this model for MuJoCo?**
Converted from URDF to MJCF with MuJoCo's compiler. Added position actuators with PD gains (kp/kv), effort limits, and joint armature; the fixed-base collision geoms are disabled (they overlap the first links).

**Are you proposing to help develop and maintain the new model?**
Yes.

Review the upstream A1_dual model at source/h1_reach/h1_reach/assets/mjcf/A1_dual and the issue's proposed conversion notes. Start with the URDF-to-MJCF conversion using MuJoCo's compiler, then verify the stated actuator, effort-limit, armature, and collision-geometry adjustments. Done means the converted and tuned A1 model is added to the Menagerie collection.

Written by the indexing model from the issue text.