Send messages to external channels through the Gateway system.

**list** — List available channels:
- operation="list"
- sessionId: Optional session ID for channel binding lookup

**send** — Send message to channel:
- operation="send"
- content: Message content (required)
- channelId: Specific channel ID (optional, auto-selects if not provided)
- sessionId: Session ID for channel lookup (optional)
- messageType: "text", "image", or "file" (default: "text")

## Usage Examples

```
# List channels
gateway_channel(operation="list")

# Send message
gateway_channel(operation="send" content="Hello!")

# Send to specific channel
gateway_channel(operation="send" channelId="telegram-main" content="Hello!")
```

## Channel Selection

If channelId not provided:
1. Use channel bound to sessionId (if provided)
2. Otherwise auto-select highest priority running channel
