clarify copilot-setup-steps.yml is required in default branch (not main)

[jancbeck]

Why:

Closes: #40511

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

Before

The copilot-setup-steps.yml workflow won't trigger unless it's present on your main branch.

after

The copilot-setup-steps.yml workflow won't trigger unless it's present on your default branch.

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.

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[Copilot]

Pull Request Overview

This PR clarifies documentation about where the copilot-setup-steps.yml workflow file must be located to trigger properly. The change corrects potentially confusing language that could mislead users about which branch is required.

• Updates documentation to specify "default branch" instead of "main branch" for clarity
• Addresses confusion about workflow trigger requirements in GitHub Copilot setup

[github-actions]

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Table of review links

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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/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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[Sharra-writes]

@jancbeck Thanks for opening an issue and PR! We can probably go ahead and do this. main is a naming convention we use that means exactly the same thing as default, but since they're interchangeable it's probably fine.

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