gram: docs

> ./examples/agent-chat-demo/.env.example

# Get your API key at https://platform.openai.com/api-keys
OPENAI_API_KEY=sk-...

# Optional: override the model (default: gpt-4o)
# OPENAI_MODEL=gpt-4o

# Comma-separated list of origins allowed to call /api/chat.
# Leave unset for local dev. SET THIS in production deployments,
# otherwise anyone hitting your URL can drive your OpenAI key.
# ALLOWED_ORIGINS=https://my-app.example.com,https://staging.example.com