fix: pin RLM_HOME outside agent workdir to survive destructive ops

[snimu]

Description

• 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

Medium Risk
Changes where rlm writes/reads session state by forcing RLM_HOME (and the metrics glob) to /tmp/.rlm, which could affect environments relying on the previous workdir-relative location.

Overview
Pins rlm session state outside the agent workdir by setting RLM_HOME to /tmp/.rlm in rlm_harness, preventing agent cleanup commands from deleting rlm’s session directory mid-rollout.

Aligns post-rollout metrics collection with this new location by updating the harness metrics_path (and related tests) to read meta.json from /tmp/.rlm/sessions/* instead of {workdir}/.rlm/....

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