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.ts

@@ -14,6 +14,7 @@ export * from "./types.hooks.js";
 export * from "./types.imessage.js";
 export * from "./types.messages.js";
 export * from "./types.models.js";
+export * from "./types.mattermost.js";
 export * from "./types.msteams.js";
 export * from "./types.plugins.js";
 export * from "./types.queue.js";