feat(env): add AI engine configuration option to .env.example and update README for improved clarity

Author: Davidson Gomes

.env.example

@@ -6,6 +6,9 @@ API_URL="http://localhost:8000"
 ORGANIZATION_NAME="Evo AI"
 ORGANIZATION_URL="https://evoai.evoapicloud.com"
 
+# AI Engine configuration: "adk" or "crewai"
+AI_ENGINE="adk"
+
 # Database settings
 POSTGRES_CONNECTION_STRING="postgresql://postgres:root@localhost:5432/evo_ai"