fix: address sonnet review MINOR findings (#2916)

- client.go: fix misleading timer.Stop() comment (finding #1)
- pr.go: document all-or-nothing semantics for GetCommitStatuses
  when check-runs endpoint fails after statuses succeed (finding #2)
- files.go: include both array and object unmarshal errors in
  ListContents fallback error message (finding #3)
- pr.go: expand mapCheckRunStatus comment to explain non-blocking
  policy decision (finding #4)

github/client.go

@@ -225,7 +225,7 @@ func (c *Client) doRequest(ctx context.Context, method, reqURL string, accept st
 				timer := time.NewTimer(delay)
 				select {
 				case <-timer.C:
-					timer.Stop() // no-op after fire, releases runtime resources promptly
+					timer.Stop() // no-op after fire; kept for symmetry with the ctx.Done case
 				case <-ctx.Done():
 					timer.Stop()
 					return nil, ctx.Err()