Fix references to the moved agent evaluation guide

[yuelgrace1810-ops]

Problem

Merged PR #2019 moved the agent-evaluation guide from docs/how-to/ to benchmarks/docs/, but two consumers retained the old path:

• tests/tooling/test_harbor_egress.py raised FileNotFoundError, making current-main make check fail.
• the mathematical benchmark README linked to the removed location.

Solution

Point both consumers at the canonical moved guide under benchmarks/docs/.

This is a path-only follow-up. It does not change Harbor behavior, network policy, benchmark contracts, or evaluation semantics.

Validation

• Exact previously failing test: 1 passed.
• make check: 1308 passed; Ruff, formatting, complexity, and mypy passed.
• Documentation command validation passed.
• Full Markdown link checker could not run locally because npx is not installed; the corrected relative target exists in this tree.

Overlap

• Direct follow-up to merged PR docs: separate benchmark material from product docs #2019.
• No open issue or PR was found owning these stale paths.

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