Variable Interpolation

ACR_API_URL and ACR_API_HEADERS support $VARIABLE interpolation from environment variables:

ACR_API_URL=https://api.example.com/v1/$ACR_MODEL/chat
ACR_API_HEADERS=Authorization: Bearer $ACR_API_KEY, X-Custom: $MY_HEADER

Missing variables are errors. Interpolation never mutates the process environment. Header overrides may use the legacy comma-separated form above or a JSON object with string values.