feat(pi): support Agent runtime export in pi_embedded backend

src/backends/piEmbedded.test.ts

@@ -66,7 +66,35 @@ describe('PiEmbeddedBackend', () => {
     try {
       const backend = new PiEmbeddedBackend({ module: mod.moduleUrl, timeoutMs: 2000 });
       await expect(backend.process({ prompt: 'hello', history: [] }))
-        .rejects.toThrow('supported session factory');
+        .rejects.toThrow('supported runtime API');
+    } finally {
+      mod.cleanup();
+    }
+  });
+
+  it('uses Agent class runtime when session factory exports are absent', async () => {
+    const mod = createModule(`
+      export class Agent {
+        constructor() {
+          this.state = { messages: [] };
+        }
+        replaceMessages(messages) {
+          this.state.messages = messages.slice();
+        }
+        async prompt(input) {
+          this.state.messages.push({ role: "user", content: [{ type: "text", text: input }] });
+          this.state.messages.push({ role: "assistant", content: [{ type: "text", text: "agent says: " + input }] });
+        }
+      }
+    `);
+
+    try {
+      const backend = new PiEmbeddedBackend({ module: mod.moduleUrl, timeoutMs: 2000 });
+      const result = await backend.process({
+        prompt: 'hello',
+        history: [{ role: 'assistant', content: 'previous answer' }],
+      });
+      expect(result).toBe('agent says: hello');
     } finally {
       mod.cleanup();
     }