move rlm secrets out of vf and into research-environments

[snimu]

Description

Move the rlm-secrets environment out of verifiers and in to research-environments, as it's used for training.

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

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Note

Medium Risk
Moderate risk because this is a full removal of an environment/package, which can break downstream users or pipelines that import or run rlm-secrets from this repo.

Overview
Removes the entire rlm-secrets environment from this repository by deleting its implementation (rlm_secrets.py), packaging metadata (pyproject.toml), and documentation (README.md).

As a result, the rlm-secrets dataset generation, tools (decrypt_position, unveil_file_number, get_code_from_file_data), and reward functions are no longer available from this codebase (presumably relocated elsewhere).

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