Build extensible agent harness with interactive chat loop

extensions/example.ts

@@ -0,0 +1,42 @@
+import type { Extension } from "../src/types.js";
+
+const exampleExtension: Extension = async ({
+  registerTool,
+  registerCommand,
+  on,
+  addSystemPrompt
+}) => {
+  await registerTool({
+    name: "echo",
+    description: "Echo text back exactly as provided.",
+    parameters: {
+      type: "object",
+      properties: {
+        text: { type: "string" }
+      },
+      required: ["text"],
+      additionalProperties: false
+    },
+    execute(input: { text: string }) {
+      return input.text;
+    }
+  });
+
+  await registerCommand({
+    name: "tools",
+    description: "Show tools registered by the example extension.",
+    execute() {
+      return "Example extension tools: echo";
+    }
+  });
+
+  addSystemPrompt("The echo tool is available when exact repetition is useful.");
+
+  on("tool.call.completed", ({ tool }) => {
+    if (tool === "echo") {
+      // Placeholder for extension-side behavior such as metrics or persistence.
+    }
+  });
+};
+
+export default exampleExtension;