Add support for all pull request operations documented in README.md that were
previously missing from the MCP server implementation. This includes:
- get_pull_request: Get details of a specific pull request
- list_pull_requests: List and filter repository pull requests
- create_pull_request_review: Create a review on a pull request
- merge_pull_request: Merge a pull request
- get_pull_request_files: Get the list of files changed in a pull request
- get_pull_request_status: Get the combined status of all status checks
- update_pull_request_branch: Update a PR branch with latest base branch changes
- get_pull_request_comments: Get the review comments on a pull request
- get_pull_request_reviews: Get the reviews on a pull request
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