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/files.go

@@ -81,7 +81,7 @@ func (c *Client) ListContents(ctx context.Context, owner, repo, path string) ([]
 	if err := json.Unmarshal(body, &entries); err != nil {
 		var single entry
 		if err2 := json.Unmarshal(body, &single); err2 != nil {
-			return nil, fmt.Errorf("parse contents JSON: %w", err2)
+			return nil, fmt.Errorf("parse contents JSON: as array: %v; as object: %w", err, err2)
 		}
 		// Guard against empty objects ({}) or unexpected shapes that
 		// unmarshal successfully but carry no useful data.