Add a GitHub MCP server tool guide

## 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.