Add src/scripts/run_policy.py

[Copilot]

run_policy.py was absent from every branch in the repository. This adds it as a first-class sibling to play.py and train.py in src/scripts/.

Changes

• src/scripts/run_policy.py — CLI script to load and execute a trained PPO policy in MuJoCo/MJX simulation
  • Reuses get_rollout / save_rollout from src/tools/rollouts.py — no duplicated rollout logic
  • Optional --save_video flag renders and saves side/back camera footage via the existing render pipeline
  • Loads env config from the run's .hydra/config.yaml, matching the pattern in train.py

Usage

# Run policy, print frame count
python src/scripts/run_policy.py --run_dir /path/to/run --checkpoint 5000000

# Run policy and save video
python src/scripts/run_policy.py --run_dir /path/to/run --checkpoint 5000000 \
    --save_video --output my_rollout --num_steps 500

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