📚 Add Gemini auth docs and improve Copilot PAT setup

[dsyme]

Summary

• Adds GEMINI_API_KEY documentation to the authentication reference page
• Refactors engine setup docs to reference the auth page instead of duplicating secret configuration instructions
• Tweaks Copilot PAT prompts and descriptions for clarity, including a help link for troubleshooting

[Copilot]

Pull request overview

This pull request improves authentication documentation by adding Gemini API key setup instructions and refactoring engine-specific auth documentation to avoid duplication. The changes also clarify Copilot PAT setup requirements in both the CLI prompts and documentation.

Changes:

• Added comprehensive GEMINI_API_KEY documentation to the authentication reference page
• Refactored engine setup docs to reference the centralized auth page instead of duplicating secret configuration instructions
• Clarified Copilot PAT prompts and descriptions, including a help link for troubleshooting

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
pkg/cli/engine_secrets.go	Simplified Copilot engine description and improved PAT setup prompts with clearer instructions and a troubleshooting help link
docs/src/content/docs/reference/engines.md	Refactored all engine sections to reference the auth page instead of duplicating setup instructions
docs/src/content/docs/reference/auth.mdx	Added new GEMINI_API_KEY section and refined COPILOT_GITHUB_TOKEN setup instructions

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[Copilot]

The "See also" reference is missing proper markdown link syntax. This should be formatted as a markdown link: [See also](/gh-aw/reference/engines/#using-google-gemini-cli) or simply as a plain link without the "See also" text, to ensure it renders correctly as a clickable link in the documentation.

[github-actions]

Hey @dsyme 👋 — thanks for improving the authentication documentation! The content here looks solid and the DRY refactoring is a great improvement. However, this PR doesn't follow the project's contribution process.

Process Issue

The CONTRIBUTING.md clearly states:

🚫 Traditional Pull Requests Are Not Enabled: You cannot create pull requests directly. Instead, you create detailed agentic plans in issues, and GitHub Copilot Agent will create and implement the PR for you after maintainer approval.

This project uses agentic development exclusively — all PRs must be created by GitHub Copilot Agent based on approved issues with detailed plans. Direct human-authored PRs bypass the workflow this project is designed to demonstrate.

What to do

1. Close this PR
2. Open an issue with an agentic plan describing these documentation improvements
3. Wait for maintainer approval
4. A maintainer will assign the issue to GitHub Copilot Agent
5. The agent will create a new PR implementing your plan

Thanks for your interest in contributing — looking forward to seeing this as an agent-implemented PR! 🤖

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