feat(cli): add generic sandbox device request flags

[cheese-head]

Summary

Add generic sandbox device request flags to the CLI by introducing --resource and --runtime-class while keeping --gpu as the single-GPU compatibility shorthand.

Related Issue

Closes #628

Changes

• Added --resource <NAME=COUNT> to openshell sandbox create
• Added --runtime-class <RUNTIME_CLASS> to openshell sandbox create
• Mapped those flags onto the existing sandbox template resources and runtime_class_name fields
• Added validation for conflicting GPU request styles
• Updated CLI tests and user-facing docs

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Notes:

• Ran cargo fmt --all
• Ran cargo test -p openshell-cli
• Verified a sandbox created with --resource nvidia.com/gpu=2 --runtime-class nvidia exposed two GPUs via nvidia-smi
• mise run pre-commit is currently blocked by an unrelated failure in openshell-sandbox:
  • process::tests::drop_privileges_succeeds_for_current_user

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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