chore(12factor-env): remove secret file support

[pyramation]

Summary

Remove filesystem-backed secret resolution from 12factor-env and standardize configuration on Kubernetes/Knative-style environment injection.

• Delete implicit /run/secrets/<name> lookup and the ${NAME}_FILE convention.
• Remove the related secret, getSecret, secretPath, getSecretsPath, and secretEnv APIs.
• Update package documentation, metadata, and dependent tests to describe/use direct environment values only.

Link to Devin session: https://app.devin.ai/sessions/1d0f77fdeaf849ea88960b9f8566fa13
Requested by: @pyramation

[devin-ai-integration]

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