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.
- 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).