feat: add Go1 training and MuJoCo deployment

docs/go1.md

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+# Go1 Adaptation
+
+The `go1` branch adds the `RobotLab-Go1-v0` IsaacLab task and a matching
+MuJoCo deployment. It uses the complete Unitree Go1 URDF/MuJoCo model.
+
+## Interface
+
+- Policy joint order: `FR, FL, RR, RL`, with hip, thigh, calf for each leg.
+- Single-frame observation: 45 values in this order:
+  body angular velocity (3), projected gravity (3), velocity command (3),
+  relative joint position (12), joint velocity (12), previous action (12).
+- Actor history: 10 frames, or 450 values.
+- Action: 12 normalized joint-position offsets, scaled by `0.25 rad` and added
+  to the default joint angles.
+- Policy frequency: 50 Hz (`0.005 s` physics step and decimation 4).
+- PD gains: `Kp=28`, `Kd=0.7`; torque is limited to `33.5 Nm`.
+- Training randomizes actuator delay over 0-4 physics steps (0-20 ms). The
+  MuJoCo config defaults to zero extra delay because its delay setting is in
+  20 ms policy steps.
+
+## Train
+
+Install the editable packages as described in the main README, then run:
+
+```bash
+cd go2_rl_robotlab
+python scripts/rsl_rl/train.py \
+  --task=RobotLab-Go1-v0 \
+  --num_envs=4096 \
+  --max_iterations=5000 \
+  --headless
+```
+
+## Play And Export
+
+`play.py` exports both `policy.pt` and `policy.onnx` into the checkpoint run's
+`exported/` directory before starting the rollout.
+
+```bash
+python scripts/rsl_rl/play.py \
+  --task=RobotLab-Go1-v0 \
+  --num_envs=64 \
+  --checkpoint=/absolute/path/to/model_5000.pt
+```
+
+## MuJoCo
+
+Place the exported TorchScript file at
+`deploy/pre_train/go1/policy.pt`, or edit `policy_path` in
+`deploy/deploy_mujoco/configs/go1.yaml`. Then run:
+
+```bash
+MUJOCO_GL=glfw python deploy/deploy_mujoco/deploy_go1.py
+```
+
+Set `xml_path` in `go1.yaml` to select `flat.xml`, `stairs.xml`, `boxes.xml`,
+or `stairs_and_slope.xml`.
+
+## Actuator Assumption
+
+The Go1 asset publishes a `33.5 Nm` effort limit but does not include measured
+torque-speed knee points. The training actuator therefore uses the known effort
+limit with delayed PD control instead of reusing the Go2-HV torque-speed curve.