Fix lint errors: handle errors, use fmt.Fprintf, apply De Morgan's law, use switch statements

Amp-Thread-ID: https://ampcode.com/threads/T-afcf56b0-a8bc-4310-bb59-1b63e1d70c89
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/renumber.go

@@ -7,7 +7,6 @@ import (
 	"os"
 	"regexp"
 	"sort"
-	"strconv"
 	"strings"
 
 	"github.com/fatih/color"
@@ -358,12 +357,6 @@ func renumberDependencies(ctx context.Context, idMapping map[string]string, allD
 	return nil
 }
 
-// Helper to extract numeric part from issue ID
-func extractNumber(issueID, prefix string) (int, error) {
-	numStr := strings.TrimPrefix(issueID, prefix+"-")
-	return strconv.Atoi(numStr)
-}
-
 func init() {
 	renumberCmd.Flags().Bool("dry-run", false, "Preview changes without applying them")
 	renumberCmd.Flags().Bool("force", false, "Actually perform the renumbering")