- Implement GetFileContentAtRef on *Client to satisfy vcs.PRReader interface
- Add escapePath and decodeBase64Content helpers
- Fix conformance_test.go to properly import and qualify github.Client
(was using unqualified Client in package github_test)
Fixes CI failure: the PRReader interface requires GetFileContentAtRef
but it was missing from this PR (only present in the file-reader PR).
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.