fix(github): address review findings from round 2880/2883

Sonnet MINOR #1: Stop timer after <-timer.C fires for idiomatic cleanup.
Sonnet MINOR #2: Document that empty array from contents API is valid (empty dir).
Sonnet MINOR #3: Document that GetPullRequestFiles returns nil for no files.
Sonnet NIT #4: Strengthen SetHTTPClient/SetRetryBackoff docs to clarify test-only intent.
Sonnet NIT #5: Document GetCommitStatuses fail-fast behavior.
Sonnet NIT #6: Document double-slash collapsing in escapePath.
Security MINOR #1: Document redirect policy responsibility when providing custom client.
Security MINOR #2: Reduce maxErrorBodyBytes from 64KB to 4KB to limit sensitive data exposure.

github/pr.go

@@ -89,6 +89,8 @@ const maxPages = 100
 
 // GetPullRequestFiles fetches the list of files changed in a PR.
 // Paginates through all pages (100 per page) to collect all files.
+// Returns nil (not an empty slice) when the PR has no changed files.
+// Callers can safely range over or check len() on a nil slice.
 func (c *Client) GetPullRequestFiles(ctx context.Context, owner, repo string, number int) ([]vcs.ChangedFile, error) {
 	var allFiles []vcs.ChangedFile
 
@@ -156,6 +158,8 @@ func (c *Client) GetFileContentAtRef(ctx context.Context, owner, repo, path, ref
 
 // GetCommitStatuses fetches both commit statuses and check runs for a SHA,
 // merging them into a unified []vcs.CommitStatus slice.
+// If the commit statuses endpoint fails (e.g. 404 for an unknown SHA), the
+// function returns immediately without attempting the check-runs endpoint.
 func (c *Client) GetCommitStatuses(ctx context.Context, owner, repo, sha string) ([]vcs.CommitStatus, error) {
 	var result []vcs.CommitStatus