test(costco): add real-world shopping evals

[walkerhughes]

Summary

• add a location-to-list eval for a two-dinner Costco run with ingredient reuse
• verify bulk package math and extended price calculations
• require hidden prices to remain unpriced instead of being treated as zero
• compare qualifying alternatives by public price
• require every MCP tool in multi-step tasks while continuing to reject endpoint bypasses

Why these evals

The original suite proves individual tools. These scenarios exercise decisions people need before a warehouse trip: where to go, how many bulk packages to buy, whether the estimate is complete, which alternative is economical, and how products can cover multiple meals.

Validation

• make check
• make coverage (87.60%)
• 57 process-check scenarios and 32 outcome-check scenarios across 8 evals
• deterministic eval generation
• git diff --check

Stack

Depends on #61, which depends on #60 and #59.