Implement PRReader conformance on the GitHub client: GetPullRequest, GetPullRequestDiff, GetPullRequestFiles (paginated, populates Patch), GetCommitStatuses (merges commit statuses + check runs). Adds compile-time PRReader conformance check. Requires PR A. Part 2 of 3 for #80.
Add GitHub API client with configurable base URL and GHE support, HTTP helpers with 429 retry and Retry-After handling. Also adds Patch field to vcs.ChangedFile. Part 1 of 3 for #80.
aweikerandclaw