refactor(agent): expose concrete harness

packages/agent/test/scratch/simple.ts

@@ -0,0 +1,13 @@
+import { getModel } from "@mariozechner/pi-ai";
+import { InMemorySessionStorage } from "../../src/harness/session/storage/memory.js";
+import { createAgentHarness, NodeExecutionEnv, Session } from "../../src/index.js";
+
+const session = new Session(new InMemorySessionStorage());
+const agent = createAgentHarness({
+	env: new NodeExecutionEnv({ cwd: process.cwd() }),
+	session,
+	initialModel: getModel("openai-codex", "gpt-5.5"),
+});
+
+const response = await agent.prompt("What is 2 + 2?");
+console.log(response);