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/client.go

@@ -199,7 +199,7 @@ func (c *Client) doRequest(ctx context.Context, method, reqURL string, accept st
 				timer := time.NewTimer(delay)
 				select {
 				case <-timer.C:
-					// Timer already fired; Stop() is a no-op here.
+					// Backoff elapsed, proceed with retry.
 				case <-ctx.Done():
 					timer.Stop()
 					return nil, ctx.Err()
@@ -259,8 +259,8 @@ func (c *Client) doRequest(ctx context.Context, method, reqURL string, accept st
 					if attempt < len(backoff) {
 						backoff[attempt] = delay
 					}
-				} else if t, err := http.ParseTime(ra); err == nil {
-					delay := time.Until(t)
+				} else if retryAt, err := http.ParseTime(ra); err == nil {
+					delay := time.Until(retryAt)
 					if delay < 0 {
 						delay = 0
 					}