feat: add GitHub MCP server to PR reviewer agent

[Copilot]

Wires the GitHub MCP server into the Copilot coding agent environment and the thorough-reviewer custom agent so both can natively call GitHub API tools (PR metadata, file diffs, code search, issues) during reviews instead of relying solely on data manually threaded through the workflow script.

Changes

• .github/copilot-mcp.json (new) — registers the GitHub MCP server via Copilot's hosted HTTP transport (https://api.githubcopilot.com/mcp/); no token wiring or sidecar container needed
• .github/agents/thorough-reviewer.agent.md — adds tools: [{type: mcp, server: github}] to the frontmatter so the agent can invoke MCP-provided GitHub tools at review time

# thorough-reviewer.agent.md (frontmatter)
tools:
  - type: mcp
    server: github

Original prompt

add na mcp server to my pr reviewer agent in this repo

The user has attached the following file paths as relevant context:

• .github/copilot-instructions.md

Created from VS Code.