examples: add smallest first agent (#3920)

Co-authored-by: Codex <codex@openai.com>

examples/first-agent/main_test.go

@@ -0,0 +1,9 @@
+package main
+
+import "testing"
+
+func TestRunFirstAgent(t *testing.T) {
+	if err := runFirstAgent(); err != nil {
+		t.Fatalf("first-agent example failed: %v", err)
+	}
+}