Add Yolo-Auto example to Copilot CLI BYOK article

[harryvgiunta]

Why:

Closes: #46018

What's being changed

Adds a short third-party gateway example to "Connecting to an OpenAI-compatible endpoint", right after the remote-OpenAI example. It uses the same three environment variables the section already introduces:

export COPILOT_PROVIDER_BASE_URL=https://yolo-auto.com/v1
export COPILOT_PROVIDER_API_KEY=YOUR-YOLO-AUTO-API-KEY
export COPILOT_MODEL=yolo

plus one sentence noting that yolo is the gateway's reasoning model and yolo-small its non-reasoning fast model, both support tool calling, streaming, and image inputs, the gateway enforces a 131072-token context window per plan, and COPILOT_PROVIDER_MAX_PROMPT_TOKENS can shrink the client budget.

Content-level only: no restructured steps, no new headings, keeps the existing {% data reusables.copilot.copilot-cli.start-cli %} step intact after the example.

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 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/copilot-cli/customize-copilot/use-byok-models.md	fpt ghec	fpt ghec

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

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[subatoi]

Closing per #46018 (comment)