Suggestion: Document job's `permissions: copilot-requests: write` as an auth mechanism

Currently the readme advises using a PAT to allow Copilot to be used: https://github.com/github/copilot-release-notes#authentication

However, for most use cases when using workflows it's a better practice to have the workflow run under the repo's budget rather than as an invidiual user (which would use their AI credits and would break if they left / lost their copilot license / etc).

Is it worth updating the root README.md with a note on using the job's permissions to enable `copilot-requests: write` for this, per guidance: https://docs.github.com/en/copilot/how-tos/copilot-cli/use-copilot-cli-in-actions#recommended-approach-github-agentic-workflows

Start with the authentication guidance in the root README.md and compare it with the linked GitHub documentation for the recommended approach. Document how a workflow job can use `permissions: copilot-requests: write` instead of a PAT, and confirm the README clearly explains when this budget-based authentication is preferable.

索引モデルが issue の本文から書いたものです。