Make Copilot auth docs easier to scan

[Copilot]

The Copilot engine docs compressed two mutually exclusive authentication paths into one dense sentence. This made CI/CD setup harder to scan and increased the chance of choosing the wrong auth path.

• Authentication guidance
  • Keeps the default engine selection note intact.
  • Splits Copilot authentication into two explicit branches:
    • organization-billed usage via copilot-requests: write
    • PAT-based usage via COPILOT_GITHUB_TOKEN

Authenticate Copilot in one of these ways:

- For organization-billed usage, grant `copilot-requests: write`.
- Otherwise, provide a `COPILOT_GITHUB_TOKEN` secret containing a fine-grained PAT with Copilot Requests access.

• Fixes [deep-report] docs: split dense auth sentence in engines/copilot.md into scannable list #54314

[Copilot]

Pull request overview

Improves Copilot authentication documentation readability and addresses #54314.

Changes:

• Preserves the default engine guidance.
• Separates organization-billed and PAT-based authentication into scannable list items.

Show a summary per file

File	Description
docs/src/content/docs/engines/copilot.md	Clarifies the two Copilot authentication paths.

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[github-actions]

🎉 This pull request is included in a new release.

Release: v0.87.2