Backward-compatible reorder: env var wins, then file. This lets
multiple instances on the same host (prod + shipyard staging)
each load a distinct token via systemd EnvironmentFile, instead
of fighting over the single ~/.secrets/bigbiggerbiggestbot file.
Also documents the new two-environment workflow in README.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>