Add support for raw GraphQL queries

跟踪现有的 list_issues GraphQL 实现和 MCP 工具注册，以了解 GitHub API 调用和参数是如何暴露的。添加支持 query 和可选 variables 的原始 GraphQL 入口，目标是文档中记载的 GraphQL 端点，并验证 queries 和 mutations 是否返回可用的结果。

Feature Request: Raw GraphQL Query Tool

Problem

The current MCP server exposes a rich set of REST-based tools but has no way to execute arbitrary GraphQL queries against the GitHub API. This is a significant gap because several important GitHub operations are only available — or significantly richer — via GraphQL.

A concrete example that motivated this request: resolving pull request review threads requires the PRRT_xxx node ID, which is only returned by GraphQL. With the current MCP server there is no way to resolve review threads programmatically, even though pull_request_review_write supports the resolve_thread method and accepts a threadId.

Proposed Solution

Add a graphql tool that executes a raw GraphQL query or mutation against https://api.github.com/graphql, e.g.:

{
  "name": "graphql",
  "description": "Execute a raw GraphQL query or mutation against the GitHub API.",
  "parameters": {
    "query": "string — the GraphQL query or mutation",
    "variables": "object (optional) — variables to pass to the query"
  }
}

Use Cases

• Fetching pull request review thread node IDs (PRRT_xxx) to resolve threads
• Querying suggestedActors to discover the Copilot bot handle before assigning
• Using assignCopilotToIssue mutation (currently only available via GraphQL)
• Any operation that returns richer data via GraphQL than the equivalent REST endpoint

Notes

• The existing list_issues tool already uses GraphQL under the hood, so the infrastructure is there
• A raw query tool would unlock the full GitHub GraphQL API surface without requiring a new dedicated tool for every GraphQL-only feature

Thanks for the great work on the MCP server!