feat: load .env file at startup using Node built-in loadEnvFile

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.

.env.example

@@ -4,5 +4,14 @@ FLYNN_TELEGRAM_TOKEN=your-bot-token-here
 # Anthropic API Key
 ANTHROPIC_API_KEY=your-anthropic-api-key-here
 
+# Optional: ZhipuAI API Key (for GLM models via /model command)
+# ZHIPUAI_API_KEY=your-zhipuai-api-key-here
+
+# Optional: OpenRouter API Key
+# OPENROUTER_API_KEY=your-openrouter-api-key-here
+
+# Optional: xAI API Key (for Grok models)
+# XAI_API_KEY=your-xai-api-key-here
+
 # Optional: Custom config path
 # FLYNN_CONFIG=/path/to/config.yaml