expose sandbox labels in SandboxEnv and CliAgentEnv

[rasdani]

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Introduces optional labeling for sandboxes to aid organization and filtering across experiments.

• Add labels: list[str] | None to SandboxEnv and CliAgentEnv constructors; pass through to CreateSandboxRequest (labels=... or [])
• Documentation updates:
  • docs/environments.md: note and example showing labels usage for SandboxEnv/CliAgentEnv
  • docs/reference.md: add SandboxEnv signature and key parameters table including labels; expand CliAgentEnv description with supported sandbox config options

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