Port model field to CustomAgentConfig from reference implementation

Adds the 'model' property to CustomAgentConfig, allowing per-agent model
selection (e.g. 'claude-haiku-4.5'). When set, the runtime will use this
model for the agent, falling back to the parent session model if unavailable.

Ports d0eb531 from github/copilot-sdk (#1309).

Co-authored-by: edburns <75821+edburns@users.noreply.github.com>

Author: Copilot

src/main/java/com/github/copilot/sdk/json/CustomAgentConfig.java

@@ -60,6 +60,9 @@ public class CustomAgentConfig {
     @JsonProperty("skills")
     private List<String> skills;
 
+    @JsonProperty("model")
+    private String model;
+
     /**
      * Gets the unique identifier name for this agent.
      *
@@ -255,4 +258,29 @@ public CustomAgentConfig setSkills(List<String> skills) {
         this.skills = skills;
         return this;
     }
+
+    /**
+     * Gets the model identifier for this agent.
+     *
+     * @return the model identifier (e.g. "claude-haiku-4.5"), or {@code null} if
+     *         not set
+     */
+    public String getModel() {
+        return model;
+    }
+
+    /**
+     * Sets the model identifier for this agent.
+     * <p>
+     * When set, the runtime will attempt to use this model for the agent, falling
+     * back to the parent session model if unavailable.
+     *
+     * @param model
+     *            the model identifier (e.g. "claude-haiku-4.5")
+     * @return this config for method chaining
+     */
+    public CustomAgentConfig setModel(String model) {
+        this.model = model;
+        return this;
+    }
 }