Move mattermost channel implementation from core to extensions/mattermost plugin. Extract config schema, group mentions, normalize utilities, and all mattermost-specific logic (accounts, client, monitor, probe, send) into the extension. Update imports to use plugin SDK and local modules. Add channel metadata directly in plugin definition instead of using getChatChannelMeta. Update package.json with channel and install configuration.
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.
- Updated message processing to include full message IDs alongside short IDs for better context resolution.
- Improved reply handling by caching inbound messages, allowing for accurate sender and body resolution without exposing dropped content.
- Adjusted tests to validate the new full ID properties and their integration into the message handling workflow.
