refactor(github): address review findings from round 2872

- client.go: clarify timer drain comment (finding #1)
- client.go: rename t -> retryAt for time.Time clarity (finding #2)
- pr.go: remove dead _ string parameter from mapCheckRunStatus (finding #3)
- files.go: add inline comment explaining zero-value guard (finding #4)

Findings #5 (NIT, no code change) and #6 (NIT, defer vs t.Cleanup
in t.Run closures) pushed back — see PR comment.

github/files.go

@@ -48,6 +48,8 @@ func (c *Client) ListContents(ctx context.Context, owner, repo, path string) ([]
 		if err2 := json.Unmarshal(body, &single); err2 != nil {
 			return nil, fmt.Errorf("parse contents JSON: %w", err2)
 		}
+		// Guard against empty objects ({}) or unexpected shapes that
+		// unmarshal successfully but carry no useful data.
 		if single.Name == "" && single.Path == "" && single.Type == "" {
 			return nil, fmt.Errorf("parse contents JSON: unexpected response format")
 		}