Preflight training smoke secret setup

[benjibc]

Summary

• add an early preflight step to training-smoke.yml that fails immediately when FIREWORKS_API_KEY is missing
• document the required and optional GitHub Actions secrets/variables for the training smoke workflow in training/README.md
• clarify that a pyroworks or pyroworks-dev key can be used for the smoke workflow

Testing

• git diff --check -- .github/workflows/training-smoke.yml training/README.md

Notes

• I could not create or upload the API key directly from this environment because mutating firectl usage is disallowed in this workspace and GitHub secret management is not available through my configured write tools.
• Current repo observation: gh secret list returns no configured Actions secrets for fw-ai/cookbook, so an admin still needs to add FIREWORKS_API_KEY in GitHub.