Improve use of GitHub MCP Server

[jerop]

We're seeing quality issues in PR review workflows, and other workflows using GitHub MCP server.

We should explore improving the use of GitHub MCP server in Gemini CLI itself.

We should also consider adding instructions for using GitHub MCP server in GEMINI.md to instruct Gemini CLI correctly. We can define the instructions in this repo, then update /setup-github to append it to user's GEMINI.md (or create it if it doesn't exist already).

[gemini-cli]

🤖 Hi @jerop, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.