Fix errcheck lint failures in runList

golangci-lint (errcheck) in CI flagged the fmt.Fprintf calls in vctl's
runList as unchecked return values — writes to an arbitrary io.Writer are
not on errcheck's default exclude list (unlike os.Stdout/os.Stderr).

- discard the fmt.Fprintf return values in runList with '_, _ =', matching
  the existing convention used elsewhere in the tree

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

vctl/main.go

@@ -241,13 +241,13 @@ func runList(w io.Writer) error {
 	}
 	names := cfg.ContextNames()
 	if len(names) == 0 {
-		fmt.Fprintf(w, "no contexts configured in %s\n", cfg.Path())
+		_, _ = fmt.Fprintf(w, "no contexts configured in %s\n", cfg.Path())
 		return nil
 	}
 	for _, name := range names {
 		rc, err := cfg.Resolve(name)
 		if err != nil {
-			fmt.Fprintf(w, "%-24s %v\n", name, err)
+			_, _ = fmt.Fprintf(w, "%-24s %v\n", name, err)
 			continue
 		}
 		status := "no token"
@@ -263,7 +263,7 @@ func runList(w io.Writer) error {
 				status = "token cached"
 			}
 		}
-		fmt.Fprintf(w, "%-24s %-40s %s\n", name, rc.Address, status)
+		_, _ = fmt.Fprintf(w, "%-24s %-40s %s\n", name, rc.Address, status)
 	}
 	return nil
 }