Make harness install timeout configurable

[snimu]

Description

The install_script timeout was hardcoded to 300s, causing RLM installs to fail in slower sandboxes. Add install_timeout to the Harness dataclass (default 300s) and default it to 600s in rlm_harness().

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

Low Risk
Small, localized change to sandbox command timeout behavior; main risk is longer waits/hanging installs if misconfigured.

Overview
Makes the agent install_script timeout configurable via a new Harness.install_timeout field (default 300s) and updates ComposableEnv to use it instead of a hardcoded 300s.

Updates rlm_harness() to accept an install_timeout parameter and defaults it to 600s to better handle slower sandbox installs.

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