Add support for raw GraphQL queries

## 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.:

```json
{
"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!