Implement Pull Request API Endpoints

This ticket covers the implementation of the core pull request-related endpoints for the GitHub Workflow MCP server. This provides the foundational functionality for the AI agent to begin interacting with pull requests.

The following endpoints have been implemented:

• GET /pull-requests
• POST /pull-requests/:pr_number/checkout
• GET /pull-requests/:pr_number/diff

Acceptance Criteria

1. A new pullRequestService.mjs is created in the services directory.
2. The service contains functions that wrap the corresponding gh CLI commands:
   • listPullRequests uses gh pr list
   • checkoutPullRequest uses gh pr checkout
   • getPullRequestDiff uses gh pr diff
3. The routes/pullRequests.mjs file is updated to call these new service functions.
4. The endpoints correctly handle requests and return the expected JSON data or plain text diff.