fix(robocerebra): drop alias extra + simplify docker image

Two problems rolled up:

1. `uv sync --locked --extra test` was failing because pyproject.toml added
   a `robocerebra = ["lerobot[libero]"]` alias extra but uv.lock wasn't
   regenerated. Drop the alias — nothing in CI actually needs the extra
   name (the Dockerfile just installs what it needs directly), so this
   restores pyproject.toml and uv.lock to byte-exact origin/main.

2. Rebase docker/Dockerfile.benchmark.robocerebra on
   huggingface/lerobot-gpu:latest (same pattern as libero/metaworld/…).
   The nightly image already ships lerobot[all] which includes [libero],
   so the RoboCerebra image is essentially identical to the LIBERO one:
   fetch libero-assets, write ~/.libero/config.yaml, overlay source.
   92 → 43 lines.

Also repoint the CI workflow comment that referenced the removed extra.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -210,7 +210,6 @@ video_benchmark = ["scikit-image>=0.23.2,<0.26.0", "pandas>=2.2.2,<2.4.0"]
 aloha = ["lerobot[dataset]", "gym-aloha>=0.1.2,<0.2.0", "lerobot[scipy-dep]"]
 pusht = ["lerobot[dataset]", "gym-pusht>=0.1.5,<0.2.0", "pymunk>=6.6.0,<7.0.0"] # TODO: Fix pymunk version in gym-pusht instead
 libero = ["lerobot[dataset]", "lerobot[transformers-dep]", "hf-libero>=0.1.3,<0.2.0; sys_platform == 'linux'", "lerobot[scipy-dep]"]
-robocerebra = ["lerobot[libero]"]  # RoboCerebra reuses the LIBERO env (libero_10 suite)
 metaworld = ["lerobot[dataset]", "metaworld==3.0.0", "lerobot[scipy-dep]"]
 
 # All