refactor: migrate messaging plugins to sdk

extensions/slack/index.ts

@@ -0,0 +1,14 @@
+import type { ClawdbotPluginApi } from "clawdbot/plugin-sdk";
+
+import { slackPlugin } from "./src/channel.js";
+
+const plugin = {
+  id: "slack",
+  name: "Slack",
+  description: "Slack channel plugin",
+  register(api: ClawdbotPluginApi) {
+    api.registerChannel({ plugin: slackPlugin });
+  },
+};
+
+export default plugin;