Enhance agent documentation by refining the definition and benefits of agents, improving clarity on YAML configuration parameters, and updating the sample interaction. Additionally, modify the Agent class to pull the model during initialization, ensuring proper setup for agent functionality.

src/main/java/io/github/ollama4j/agent/Agent.java

@@ -137,6 +137,7 @@ public class Agent {
             }
             Ollama ollama = new Ollama(agentSpec.getHost());
             ollama.setRequestTimeoutSeconds(120);
+            ollama.pullModel(agentSpec.getModel());
             return new Agent(
                     agentSpec.getName(),
                     ollama,