Document default retry rules applied during agent execution

## Overview
AgentKit applies retry logic during agent execution, but the default rules are not clearly documented. Developers may need to inspect implementation details to understand when and how retries occur.

## Problem Description
Without clear guidance, it is difficult to know which failures trigger retries, how many retry attempts are made, or how retries affect execution flow and timing.

## Expected Behavior
The documentation should clearly describe the conditions under which retries are performed and how retry limits or backoff are applied.

## Proposed Solution
Add a short documentation section that explains the default retry rules, including which errors are retryable, how many attempts are made, and how retries impact agent execution.