Adds JSON output mode for that returns QR code
as base64 PNG data URL instead of terminal graphics. Useful for web
dashboards and hosting platforms that need to display the QR in a browser.
- Add --json and --timeout flags to channels login command
- Use gateway.loginWithQrStart/Wait methods for JSON mode
- Output two JSON objects: QR data, then connection result
- Document new mode in docs/cli/channels.md
Add Mattermost as a supported messaging channel with bot API and WebSocket integration. Includes channel state tracking (tint, summary, details), multi-account support, and delivery target routing. Update documentation and tests to include Mattermost alongside existing channels.
