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/mattermost/index.ts

@@ -0,0 +1,9 @@
+export {
+  listEnabledMattermostAccounts,
+  listMattermostAccountIds,
+  resolveDefaultMattermostAccountId,
+  resolveMattermostAccount,
+} from "./accounts.js";
+export { monitorMattermostProvider } from "./monitor.js";
+export { probeMattermost } from "./probe.js";
+export { sendMessageMattermost } from "./send.js";