fix: enforce strict config validation

extensions/matrix/index.ts

@@ -1,4 +1,5 @@
 import type { ClawdbotPluginApi } from "clawdbot/plugin-sdk";
+import { emptyPluginConfigSchema } from "clawdbot/plugin-sdk";
 
 import { matrixPlugin } from "./src/channel.js";
 import { setMatrixRuntime } from "./src/runtime.js";
@@ -7,6 +8,7 @@ const plugin = {
   id: "matrix",
   name: "Matrix",
   description: "Matrix channel plugin (matrix-js-sdk)",
+  configSchema: emptyPluginConfigSchema(),
   register(api: ClawdbotPluginApi) {
     setMatrixRuntime(api.runtime);
     api.registerChannel({ plugin: matrixPlugin });