✨ feat(cassette): declare config environment mappings

[jpmcb]

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Closes #322.

Summary

• add optional env mappings to cassette config entries
• derive omitted mappings from the uppercased key with dots converted to underscores
• validate, canonicalize, and publish resolved environment variable names
• update hello-world to use GREETING and document the manifest behavior

Validation

• make format
• make test
• independent reviewer: no findings