Add manifest for creating custom app

[aliciatang07]

add manifest for custom app

[gemini-cli]

📋 Review Summary

This PR introduces a GitHub App manifest file to facilitate the creation of a custom GitHub App for using this action. While this is a positive step towards simplifying setup, the initial configuration requests overly broad permissions and contains placeholder values that need clarification.

🔍 General Feedback

• The addition of a manifest file is a great idea for improving the user experience.
• The manifest is well-structured and includes helpful comments.
• The primary area for improvement is adhering to the principle of least privilege for permissions.

🎯 Specific Feedback

🟡 High

• File: custom_app_manifest.yaml:14-16 - The permissions requested (contents: write, issues: write, pull_requests: write) are overly permissive for the action's described functionality. The action primarily runs the Gemini CLI and does not appear to need write access to repository contents, issues, or pull requests. To follow the principle of least privilege, these should be reduced. I recommend contents: read if the CLI needs to read files from the repo, and removing issues and pull_requests permissions entirely if they are not used.

🟢 Medium

• File: custom_app_manifest.yaml:6 - The name field contains a placeholder "orgname". It should be made clear in the documentation or comments that users must replace this with their actual organization or a unique name.
• File: custom_app_manifest.yaml:7 - The homepage_url is set to a placeholder https://example.com. This should be updated by the user, and it would be helpful to note this in the associated documentation.

🔵 Low

• File: custom_app_manifest.yaml:23 - The file is missing a final newline character, which is a common convention for text files.

✅ Highlights

• Adding a custom_app_manifest.yaml is an excellent way to help users configure a GitHub App quickly and correctly.
• The inclusion of comments explaining the purpose of different fields is very helpful.

[jerop]

how will this manifest be used? please add docs

[JeromeJu]

LGTM