feat: add stepDelay config option

Add configurable delay between agent steps.
Previously hardcoded to 0.4s.

Changes:
- Add stepDelay?: number to AgentConfig
- Use config value with 0.4s default

packages/core/src/PageAgentCore.ts

@@ -346,7 +346,7 @@ export class PageAgentCore extends EventTarget {
 				return result
 			}
 
-			await waitFor(0.4) // @TODO: configurable
+			await waitFor(this.config.stepDelay ?? 0.4)
 		}
 	}