Implement FileReader conformance on the GitHub client: GetFileContent,
ListContents, path helpers, base64 decode. Includes compile-time
conformance checks for both PRReader and FileReader.
Requires PR B (#102). Part 3 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.