ci(trader-joes): gate MCP evals in Modal

[walkerhughes]

Depends on #56.

Summary

• add four deterministic Harbor tasks for store lookup, price search, SKU lookup, and shopping-list pricing
• score both outcome correctness and required MCP use
• detect direct endpoint bypasses in main and delegated agent transcripts
• run the Claude Code agent in Modal as a serialized CI merge gate
• pin eval images to the PR head SHA and upload trial artifacts for diagnosis

Validation

• Harbor resolves all four tasks with the Modal environment
• 28 synthetic process-verifier cases pass
• reward gate self-test passes
• full plugin suite passes with 92% coverage