feat(pi): capture subagent activity events

modules/agents/pi/extensions/subagents/runner.test.ts

@@ -26,6 +26,55 @@ function events(): RunnerEvents {
   };
 }
 
+test("child RPC process forwards structured activity before collecting the final result", async (t) => {
+  const running: unknown[] = [];
+  const completed: Array<{ result: string; stopReason?: string }> = [];
+  const fakeChild = new EventEmitter() as EventEmitter & {
+    stdout: FakeStream;
+    stderr: FakeStream;
+    stdin: FakeStream;
+    killed: boolean;
+    pid?: number;
+    kill(signal?: NodeJS.Signals): boolean;
+  };
+  fakeChild.stdout = new FakeStream();
+  fakeChild.stderr = new FakeStream();
+  fakeChild.stdin = new FakeStream();
+  fakeChild.killed = false;
+  fakeChild.kill = () => {
+    fakeChild.killed = true;
+    return true;
+  };
+  t.mock.method(fakeChild.stdin, "write", (chunk, callback?: (error?: Error | null) => void) => {
+    const request = JSON.parse(String(chunk)) as { id: string; type: string };
+    callback?.();
+    if (request.type === "get_last_assistant_text") {
+      queueMicrotask(() => {
+        fakeChild.stdout.emit("data", `${JSON.stringify({ id: request.id, type: "response", success: true, data: { text: "final answer" } })}\n`);
+      });
+    }
+    return true;
+  });
+  t.mock.method(childProcess, "spawn", () => fakeChild as unknown as childProcess.ChildProcessWithoutNullStreams);
+
+  const { SubprocessRpcRunner } = await import("./runner.ts");
+  const runner = new SubprocessRpcRunner();
+  await runner.start("child-1", { prompt: "work", label: "Review migration" }, "/tmp", {
+    ...events(),
+    running: (event) => running.push(event),
+    completed: (result, stopReason) => completed.push({ result, stopReason }),
+  });
+
+  const firstActivity = { type: "message_start", role: "assistant", message: { id: "msg-1" } };
+  const secondActivity = { type: "tool_execution_start", tool: "read", input: { path: "runner.ts" } };
+  const settledActivity = { type: "agent_settled" };
+  fakeChild.stdout.emit("data", `${JSON.stringify(firstActivity)}\n${JSON.stringify(secondActivity)}\n${JSON.stringify(settledActivity)}\n`);
+  await new Promise((resolve) => setImmediate(resolve));
+
+  assert.deepEqual(running, [firstActivity, secondActivity, settledActivity]);
+  assert.deepEqual(completed, [{ result: "final answer", stopReason: "agent_settled" }]);
+});
+
 test("child RPC process disables discovery while explicitly loading subagents extension", async (t) => {
   const calls: Array<{ command: string; args: string[] }> = [];
   const fakeChild = new EventEmitter() as EventEmitter & {