Add RLM monitor rubric

[snimu]

Description

Adds RLMMonitorMetric so that the individual environments don't have to implement it

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

Adds a shared rubric to standardize RLM monitoring across environments.

• New RLMMonitorRubric in verifiers/envs/experimental/rlm_env.py exposes metrics for sub_llm_* (calls, turns, tokens, tool calls, batch stats), main_rlm_* (turns, tokens), and REPL timing (repl_*)
• RLMEnv.__init__ now registers the rubric via add_rubric(RLMMonitorRubric()) for automatic metric collection

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