feat: add Mattermost channel support

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.

src/config/types.queue.ts

@@ -13,6 +13,7 @@ export type QueueModeByProvider = {
   telegram?: QueueMode;
   discord?: QueueMode;
   slack?: QueueMode;
+  mattermost?: QueueMode;
   signal?: QueueMode;
   imessage?: QueueMode;
   msteams?: QueueMode;