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
 					}

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")
 		}

github/pr.go

@@ -194,7 +194,7 @@ func (c *Client) GetCommitStatuses(ctx context.Context, owner, repo, sha string)
 		for _, cr := range checkResp.CheckRuns {
 			result = append(result, vcs.CommitStatus{
 				Context:     cr.Name,
-				Status:      mapCheckRunStatus(cr.Conclusion, cr.Status),
+				Status:      mapCheckRunStatus(cr.Conclusion),
 				Description: derefString(cr.Conclusion),
 				TargetURL:   cr.HTMLURL,
 			})
@@ -208,10 +208,9 @@ func (c *Client) GetCommitStatuses(ctx context.Context, owner, repo, sha string)
 }
 
 // mapCheckRunStatus maps a check run conclusion to a vcs.CommitStatus status string.
-// The second parameter (check run status field, e.g. "completed", "in_progress") is
-// unused because conclusion alone determines the mapped state: nil conclusion means
-// the run is still in progress (pending), regardless of the status field value.
-func mapCheckRunStatus(conclusion *string, _ string) string {
+// Conclusion alone determines the mapped state: nil conclusion means the run is
+// still in progress (pending), regardless of the status field value.
+func mapCheckRunStatus(conclusion *string) string {
 	if conclusion == nil {
 		// Still running or queued
 		return "pending"