solutions-engineering: harden integration debugging path

[adityamaru]

Summary

• Rename the integration test runner and env vars so the root cause is less obvious from static inspection.
• Keep CI output generic while pointing candidates toward runtime service logs for evidence.
• Add a migration-lab CLAUDE.md to keep Claude/Codex scoped to the SE exercise and away from other interview directories.

Test plan

• Ran npm run lint in solutions-engineering/migration-lab/app.
• Ran the Docker-backed integration command locally and verified CI output is generic while docker compose logs postgres shows sorry, too many clients already.

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