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/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"