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.

extensions/mattermost/index.ts

@@ -0,0 +1,18 @@
+import type { ClawdbotPluginApi } from "clawdbot/plugin-sdk";
+import { emptyPluginConfigSchema } from "clawdbot/plugin-sdk";
+
+import { mattermostPlugin } from "./src/channel.js";
+import { setMattermostRuntime } from "./src/runtime.js";
+
+const plugin = {
+  id: "mattermost",
+  name: "Mattermost",
+  description: "Mattermost channel plugin",
+  configSchema: emptyPluginConfigSchema(),
+  register(api: ClawdbotPluginApi) {
+    setMattermostRuntime(api.runtime);
+    api.registerChannel({ plugin: mattermostPlugin });
+  },
+};
+
+export default plugin;