Adds process.loadEnvFile() to CLI entry point so API keys (ZHIPUAI_API_KEY,
OPENROUTER_API_KEY, XAI_API_KEY, etc.) can be stored in a project .env file
instead of shell environment or systemd service config. Uses Node >= 20.12
built-in — no dotenv dependency needed. Silent no-op if .env doesn't exist.
Updates .env.example with placeholders for all provider API keys.