feat: publish motrixlab metapackage with builtin RL and MotrixSim

[wlgys8]

Summary

• Add a metadata-only motrixlab workspace package, pinning env-core, deploy, RL, builtin RL, and the MotrixSim environment backend to the same release.
• Extend the PyPI/TestPyPI workflow to six wheels/publishers; publish the aggregate after both new components.
• Update installation/release documentation, lockfile and the outdated internal-PyPI note.

Stacking / issue

Refs #132. Prerequisite #133 has merged. This branch has been rebased onto main and the PR targets main; the diff contains only the aggregate package changes.

Validation

• prek run --all-files, git diff --check, uv lock --check: passed.
• tests/test_workspace_versions.py: 3 passed.
• Six wheels built and passed twine check; aggregate sdist-to-wheel roundtrip passed. Aggregate wheel contains only dist-info and the expected five exact dependencies.
• Linux x86_64 clean Python 3.10.18 / 3.11.12 environments: actual install of six local wheels plus public-PyPI third-party dependencies (70 packages each), dependency check, imports, builtin provider discovery (framework name motrix), default MotrixSim registration/backend class loading, and mxtrain/mxplay/mxexport --help all passed outside the checkout.
• Existing workspace regression command failed during collection because its Torch environment lacks libcudnn.so.9. No GPU simulation/training is claimed.

Release prerequisites still pending

• The three GitHub environments have been created: pypi-rl-builtin, pypi-env-motrixsim, pypi-motrixlab.
• Maintainer must configure Pending/Trusted Publishers on both PyPI and TestPyPI for the three new projects; public JSON queries return 404, which does not guarantee name registration.
• Actual workflow publication and installation from published indexes remain release acceptance steps. No release/tag/upload performed; issue intentionally remains open.

Full validation and external setup details are recorded on #132.