feat!: move G1 walk rough downstream

[TATP-233]

Summary

• remove the G1WalkRough registry entry, SAC owners, task-specific rough-scene assets, tests, benchmark target, and support claim from UniLab
• update core docs to retain only UniLab-owned task examples and describe terrain capability evidence without relying on the downstream Unitree owner
• ownership now lives in unilabsim/unitree_rl_unilab commit 3aff9ac

Validation:

• make check
• make test-all (1479 passed, 26 skipped, 648 deselected; benchmark smoke passed)
• focused slow suites: G1 owner contract, locomotion params, offpolicy train scripts/configs
• uv run pytest tests/scripts/test_check_docs.py -q
• Sphinx build with UNILAB_DOCS_SKIP_AUTODOC=1

Follow-up to #1576 and #1583.

BREAKING CHANGE: G1WalkRough and g1_walk_rough are no longer registered by core UniLab; use downstream UnitreeG1WalkRough / unitree_g1_walk_rough.