Add Gemini CLI extension

[marzvrover]

Adds a gemini-extension.json manifest file so that the remote MCP server can be installed with the gemini extensions install https://github.com/github/github-mcp-server command.

Also updates the env var name for the PAT in all Gemini CLI cases to be GITHUB_MCP_PAT. Gemini CLI uses environment variables plus those found in the .env file, so I caution using a more explicitly named variable to prevent a user accidentally using a PAT they have in their environment that contains permissions that they may not want their MCP instance to have access to.

[Copilot]

Pull Request Overview

This PR adds support for installing the GitHub MCP server via Gemini CLI's extension system and standardizes environment variable naming for better security. The changes introduce a gemini-extension.json manifest file that enables one-command installation and updates all documentation to use the more explicit GITHUB_MCP_PAT environment variable name instead of the generic GITHUB_PAT.

• Added gemini-extension.json manifest file for Gemini CLI extension support
• Updated environment variable name from GITHUB_PAT to GITHUB_MCP_PAT across all installation methods
• Simplified installation documentation to recommend the new extension-based approach

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
gemini-extension.json	New manifest file defining the GitHub MCP server extension configuration
docs/installation-guides/install-gemini-cli.md	Updated to use new env var name and recommend extension installation method

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[tonytrg]

left some comments, but tested the flow and it works nicely great stuff!