add test for SetSessionID preamble update

pkg/agent/core_test.go

@@ -1705,6 +1705,33 @@ func TestSetSessionID_Rename(t *testing.T) {
 	}
 }
 
+func TestSetSessionID_UpdatesPreamble(t *testing.T) {
+	oldID := NewSessionID()
+	env := AgentEnv{
+		preamble: "session is " + oldID + " end",
+	}
+	c := NewAgentCore(AgentCoreConfig{
+		Backend:       defaultBE(),
+		AgentName:     "test",
+		AgentsDir:     t.TempDir(),
+		SessionsDir:   t.TempDir(),
+		SessionID:     oldID,
+		CWD:           t.TempDir(),
+		Env:           env,
+		NewDispatcher: tools.NewDispatcher,
+	}).(*agent)
+	t.Cleanup(c.Close)
+
+	newID := NewSessionID()
+	if err := c.SetSessionID(newID); err != nil {
+		t.Fatalf("SetSessionID: %v", err)
+	}
+	want := "session is " + newID + " end"
+	if got := c.SystemPrompt(); got != want {
+		t.Errorf("SystemPrompt() = %q; want %q", got, want)
+	}
+}
+
 func TestSetSessionID_SameID(t *testing.T) {
 	c := newCore(t, nil, nil)
 	id := c.sessionID