feat(cli): add slurm resource attr export

[rhewett-nv]

What does this PR do?

Adds nemo-lens set-slurm-resource-attrs, an eval-friendly CLI for appending
Lens-derived Slurm launch resource attributes to OTEL_RESOURCE_ATTRIBUTES.

Issues

N/A

Usage

eval "$(nemo-lens set-slurm-resource-attrs --stage sbatch)"

srun ... bash -lc '
  eval "$(nemo-lens set-slurm-resource-attrs \
    --stage task \
    --container-image "$CONTAINER_IMAGE")"
  exec "$@"
'

Before your PR is "Ready for review"

Pre checks:

• I read the Contributor guidelines.
• All commits are signed off (git commit -s) per the DCO.
• I added or updated tests under tests/ for any behavior change.
• I ran pytest and pre-commit run --all-files locally; both pass.
• I updated docs/ and README.md where behavior, configuration, or public API changed.
• If this changes a lens public API symbol, I updated the cross-repo _fallbacks.py files in Megatron-LM / RL / Gym.

Additional information

Validation on pr5-slurm-resource-attrs-cli:

• ../.venv/bin/pre-commit run --all-files: passed
• PYTHONPATH=src ../.venv/bin/python -m pytest -q: 265 passed, 7 warnings
• ../.venv/bin/python -m compileall -q src tests: passed
• npm --prefix docs/fern run check: passed with 0 errors; one warning because the unauthenticated redirects check is skipped without fern login / FERN_TOKEN

Notes:

• PR5 is one signed commit: eb98291 feat(cli): add slurm resource attr export.
• The command prints one shell export for OTEL_RESOURCE_ATTRIBUTES.
• Existing OTEL_RESOURCE_ATTRIBUTES values are preserved and take precedence
  over locally derived fallback values.
• --stage sbatch captures scheduler/script-context Slurm attributes such as
  slurm.head_node.name when Slurm exposes them.
• --stage task captures task-local attributes such as host.name,
  slurm.topology.*, and optional nv.dl.launch.container.image.
• --container-image is valid only with --stage task; sbatch usage returns 2.
• New SDK-specific imports are not introduced, no frozen
  docs/fern/versions/0.1.0/ pages are changed, and no top-level Lens API or
  fallback surface changes are required.

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[ahmadki]

/ok to test e151da0

The merge-base changed after approval.