Include commit hash in environment variables

When deployed from GitHub, the commit hash should be included as an environment variable.

For logging/reporting purposes, it's helpful to be able to link your deployment to a specific commit in the repository.

The issue names GitHub deployments and environment variables but no repository file, test, or entry point. First locate where deployment environment variables are defined, then determine how a deployed revision can expose its commit hash and verify that the value supports logging and reporting.

Written by the indexing model from the issue text.