Update link for customizing the agent's firewall

[knownasnaffy]

Why:

Invalid link pointing to the void (non-existing page) that should've pointed to "Customizing the agent firewall" page.

Closes: NA

What's being changed (if available, include any code snippets, screenshots, or gifs):

Fixed the the link to point to the right document.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

How to review these changes 👓

Thank you for your contribution. To review these changes, choose one of the following options:

• Spin up a codespace
• Set up a local development environment

A Hubber will need to deploy your changes internally to review.

Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
copilot/how-tos/copilot-on-github/customize-copilot/customize-cloud-agent/customize-the-agent-environment.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

🤖 This comment is automatically generated.

[Copilot]

Pull request overview

This PR updates an internal documentation link in the Copilot cloud agent “Configure the development environment” article to point readers to the agent firewall documentation.

Changes:

• Replaced a previously invalid firewall link with a new URL for the firewall documentation.