Update documentation to replace subAgent with agentStart and agentMessage

- Replace references to the old subAgent tool with the new agentStart and agentMessage tools\n- Update the Available Tools section to reflect the new asynchronous agent workflow

Author: bhouston

docs/usage/index.mdx

@@ -144,7 +144,8 @@ MyCoder has access to a variety of tools that enable it to perform complex tasks
 | **fetch**            | Makes HTTP requests                              | Accessing APIs, downloading resources                            |
 | **browseStart**      | Starts a browser session                         | Researching documentation, exploring solutions                   |
 | **browseMessage**    | Performs actions in an active browser            | Navigating websites, extracting information                      |
-| **subAgent**         | Creates specialized sub-agents                   | Handling complex tasks in parallel                               |
+| **agentStart**       | Starts a sub-agent and returns immediately       | Creating asynchronous specialized agents for parallel tasks      |
+| **agentMessage**     | Interacts with a running sub-agent               | Checking status, providing guidance, or terminating sub-agents   |
 
 For more detailed information about specific features, check the following pages: