Add a GitHub MCP server tool guide

[oto-macenauer-absa]

Summary

Add a guide for the GitHub MCP server in the docs/tools/ section. Our work — repositories, issues, and pull requests — lives in GitHub, and the official GitHub MCP server lets agents read repos and manage issues/PRs directly. It pairs naturally with our existing create-issue and pr-review skills.

The guide should cover

• What the GitHub MCP server is and what it enables (repos, issues, PRs).
• How to set it up for our agents (GitHub Copilot CLI primarily; note others).
• Cost-aware configuration — tool schemas are input tokens on every turn, so cover toolset scoping, read-only vs write, and remote vs local/Docker modes.
• When to connect it vs. leave it off.
• How it complements the create-issue / pr-review skills.

Notes

• Back the findings with web research across multiple independent sources.
• Keep it tool-general where possible; be honest about the token overhead and how to minimise it.