feat: support configuring a default_agent across all API/user surfaces (#5843)

Co-authored-by: observerw <observerw@users.noreply.github.com>

packages/sdk/js/src/v2/gen/types.gen.ts

@@ -1417,6 +1417,10 @@ export type Config = {
    * Small model to use for tasks like title generation in the format of provider/model
    */
   small_model?: string
+  /**
+   * Default agent to use when none is specified. Must be a primary agent. Falls back to 'build' if not set or if the specified agent is invalid.
+   */
+  default_agent?: string
   /**
    * Custom username to display in conversations instead of system username
    */
@@ -1770,6 +1774,7 @@ export type Agent = {
   mode: "subagent" | "primary" | "all"
   native?: boolean
   hidden?: boolean
+  default?: boolean
   topP?: number
   temperature?: number
   color?: string