ci(costco): gate MCP evals in Modal

[walkerhughes]

Summary

• add four deterministic Harbor tasks covering warehouse discovery, catalog search, item lookup, and shopping-list pricing
• score outcomes separately from MCP-use and endpoint-bypass process checks
• run the full agent loop in Modal as a required Costco CI job
• pin Harbor 0.18.0 and install the exact PR SHA inside each eval image

Validation

• make check
• make coverage (87.60%)
• harbor run ... --print-config with the Modal environment
• official plugin and skill validators
• workflow YAML parse

Stack

Depends on #60, which depends on #59.