fix(corpus): normalize naive staleness timestamps

[Sanjays2402]

Description

Normalize timezone-naive document metadata timestamps to UTC before comparing them with the staleness cutoff, preventing a TypeError during corpus audits.

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
• Refactoring (no functional changes)
• Test update
• CI/CD update

Related Issues

Closes #60

How Has This Been Tested?

Added a regression test using a naive datetime; it fails on main and passes with this fix. All 10 staleness tests pass.

• Unit tests pass (uv run pytest)
• Linter passes (uv run ruff check .)
• Type checker passes (uv run mypy openagent_eval/)
• Manual testing performed

Checklist

• My code follows the project's coding standards
• 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
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

N/A

Additional Notes

Changed-file Ruff findings and formatting results are unchanged from main.

[github-actions]

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