Update Copilot Setup Steps for use with a merge queue

[jonstacks]

Why:

Closes: #43246

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

When I told copilot the issue I was having, it went for updating with a note. If the GitHub team decides its better implemented some other way, that works too!

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]

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Source	Review	Production	What Changed
copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment.md	fpt ghec	fpt ghec

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

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[Copilot]

Pull request overview

Updates the Copilot Coding Agent documentation to address how copilot-setup-steps.yml workflow triggers behave when a repository uses a merge queue, aiming to reduce unnecessary workflow runs.

Changes:

• Adds a new note explaining that merge queues can cause additional push-triggered workflow runs.
• Suggests workflow trigger filters (e.g., ignoring gh-readonly-queue/**) to avoid merge-queue-related runs and links to merge queue documentation.

[Copilot]

The mitigation here implies both branches-ignore (on push) and restricting pull_request to the default branch are needed to stop merge-queue-related runs. In practice, the extra runs come from the merge queue pushing to gh-readonly-queue/**, so branches-ignore on push addresses that; tightening pull_request doesn’t prevent the merge-queue push-triggered runs and could unintentionally stop validation for PRs targeting non-default branches. Consider rewording to make the pull_request.branches restriction optional/separate, and (if relevant) call out that workflows required for merge queue checks should use the merge_group event instead of relying on push to queue branches.

Suggested change

> If your repository uses a merge queue, the `push` trigger above will also run when pull requests are added to the merge queue. To prevent this, add `branches-ignore: ['gh-readonly-queue/**']` to the `push` trigger and restrict the `pull_request` trigger to your default branch with `branches: [YOUR-DEFAULT-BRANCH]`. For more information, see [AUTOTITLE](/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue).

> If your repository uses a merge queue, the `push` trigger above will also run when pull requests are added to the merge queue, because the merge queue pushes to `gh-readonly-queue/**` branches. To prevent these extra runs, add `branches-ignore: ['gh-readonly-queue/**']` to the `push` trigger. If you only want to validate pull requests that target your default branch, you can additionally restrict the `pull_request` trigger with `branches: [YOUR-DEFAULT-BRANCH]`, but this is optional and will stop validation for pull requests targeting other branches. For workflows that must participate in merge queue checks, consider using the `merge_group` event instead of relying on `push` to queue branches. For more information, see [AUTOTITLE](/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue).