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/utils/message-channel.ts

@@ -22,6 +22,7 @@ const MARKDOWN_CAPABLE_CHANNELS = new Set<string>([
   "telegram",
   "signal",
   "discord",
+  "mattermost",
   "tui",
   INTERNAL_MESSAGE_CHANNEL,
 ]);