(fix): align Campaign Pack product contract

[kyeunga25]

Summary

• add the required product category field to the private Campaign Pack brief
• keep workspace readiness and Campaign Agent validation aligned with the Worker payload contract
• add focused regression coverage for the field and missing-category state

Verification

• npm run check
• npm test (67 tests)
• npm run build
• npm audit --audit-level=high
• npm run cf:dry-run
• npm run release:check
• git diff --check