Add copilot-setup-steps.yml to configure GitHub Copilot environment

[Copilot]

• Create .github/workflows/copilot-setup-steps.yml file with the required configuration
• Ensure the workflow follows the structure from the issue description
• Test that the workflow file is valid YAML and follows GitHub Actions syntax
• Verify the workflow is properly configured for the wp-cli organization's needs
• Add copilot-setup-steps.yml to sync-workflows.yml for distribution to all repositories
• Add composer.json existence check to prevent running PHP/Composer steps in non-PHP projects

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add copilot-setup-steps.yml file to all repositories</issue_title>
<issue_description>As per the docs:

In its ephemeral development environment, Copilot can build or compile your project and run automated tests, linters and other tools. To do this, it will need to install your project's dependencies.

Copilot can discover and install these dependencies itself via a process of trial and error, but this can be slow and unreliable, given the non-deterministic nature of large language models (LLMs), and in some cases, it may be completely unable to download these dependencies—for example, if they are private.

Instead, you can preconfigure Copilot's environment before the agent starts by creating a special GitHub Actions workflow file, located at .github/workflows/copilot-setup-steps.yml within your repository.

A copilot-setup-steps.yml file looks like a normal GitHub Actions workflow file, but must contain a single copilot-setup-steps job. This job will be executed in GitHub Actions before Copilot starts working.

It could look like this:

name: "Copilot Setup Steps"

on:
  workflow_dispatch:
  push:
    paths:
      - .github/workflows/copilot-setup-steps.yml
  pull_request:
    paths:
      - .github/workflows/copilot-setup-steps.yml

jobs:
  copilot-setup-steps:
    runs-on: ubuntu-latest
    permissions:
      contents: read

    steps:
      - name: Checkout code
        uses: actions/checkout@v5

      - name: Set up PHP environment
        uses: shivammathur/setup-php@v2
        with:
          php-version: 'latest'
          ini-values: zend.assertions=1, error_reporting=-1, display_errors=On
          coverage: 'none'
          tools: composer,cs2pr
        env:
          COMPOSER_TOKEN: ${{ secrets.GITHUB_TOKEN }}

      - name: Install Composer dependencies & cache dependencies
        uses: ramsey/composer-install@v3
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Add copilot-setup-steps.yml file to all repositories #169

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[swissspidy]

@copilot Add a new "Check existence of composer.json file" step like in other workflows, for some extra hardening.