Add subdomain specification documentation for AI agent

ai/agent.mdx

@@ -21,7 +21,7 @@
 ## Add the agent to your Slack workspace
 
 <Note>
   If your Slack Workspace Owner requires admin approval to install apps, ask them to approve the Mintlify app before you connect it.
 </Note>
 
 1. Navigate to the [agent](https://dashboard.mintlify.com/products/agent) page of your dashboard.
@@ -56,6 +56,17 @@
 - `@mintlify Find and fix all typos in the docs`
 - `@mintlify change all unordered lists to use * instead of -`
 
+## Specify a subdomain
+
+If you have multiple documentation sites with their own subdomains, include the `subdomain` parameter in your message to specify which subdomain the agent should work on.
+
+Use the format `@mintlify subdomain=<your-subdomain> <your-prompt>` to prompt the agent to work on a specific subdomain.
+
+Examples:
+
+- `@mintlify subdomain=public-docs Add a new section to the quickstart about inviting collaborators based on this PR`: Prompts the agent to update the quickstart only on the `public-docs` subdomain.
+- `@mintlify subdomain=customer-docs Update the auth docs for the new authentication method`: Prompts the agent to update the auth docs only on the `customer-docs` subdomain.
+
 ## Agent workflows
 
 The agent assists with many different documentation tasks. These workflows show some of the ways you can integrate the agent into your documentation process. Try an approach that fits how your team currently works and adapt it to your specific needs.