fix(lint): suppress errcheck on fmt.Fprintf in setInitUsage

golangci-lint's errcheck flagged the unchecked return value of
fmt.Fprintf(w, ...) inside the setInitUsage closure. Add the
standard _, _ = discard prefix to match the rest of the file.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

cmd/mdsmith/init.go

@@ -23,7 +23,7 @@ import (
 func setInitUsage(fs *flag.FlagSet, w io.Writer) {
 	fs.SetOutput(w)
 	fs.Usage = func() {
-		fmt.Fprintf(w,
+		_, _ = fmt.Fprintf(w,
 			"Usage: mdsmith init [--starter <name>] [--from-markdownlint[=path]] [--add <pack>] [--force] [--list]\n\n"+
 				"Write .mdsmith.yml in the current directory and, optionally, scaffold\n"+
 				"additive .mdsmith/ packs beside it.\n\n"+