Make code lint-clean under the canonical golangci-lint config

150 findings fixed: linter autofixes for whitespace style (wsl_v5,
nlreturn, noinlineerr), named returns removed, magic numbers replaced
with named constants, static errNotRegular error, explicit Close/Parse
error handling, unused cobra params renamed to _, and runReport/
runTrees/hashPass split into helpers to satisfy cyclop, gocognit, and
funlen. Behavior verified unchanged against the README smoke test.

main.go

@@ -20,15 +20,23 @@ import (
 	"github.com/spf13/cobra"
 )
 
+// Exit codes: 0 is success (even with per-file warnings), exitFatal is
+// a fatal error, exitUsage is a usage error.
+const (
+	exitFatal = 1
+	exitUsage = 2
+)
+
 func main() {
 	root := &cobra.Command{
 		Use:   "sfdupes",
 		Short: "Find candidate duplicate files by size and head/tail SHA-256",
 		Args:  cobra.NoArgs,
-		Run: func(cmd *cobra.Command, args []string) {
+		Run: func(cmd *cobra.Command, _ []string) {
 			// A missing subcommand prints usage and exits 2.
 			_ = cmd.Usage()
-			os.Exit(2)
+
+			os.Exit(exitUsage)
 		},
 	}
 	// Everything on stdout is machine-readable data; all human-facing
@@ -43,7 +51,7 @@ func main() {
 		// The README specifies single-dash flags (-root, -workers);
 		// parse them with the stdlib flag package inside runScan.
 		DisableFlagParsing: true,
-		Run: func(cmd *cobra.Command, args []string) {
+		Run: func(_ *cobra.Command, args []string) {
 			runScan(args)
 		},
 	}
@@ -52,7 +60,7 @@ func main() {
 		Use:   "report [files.dat|-]",
 		Short: "Read a scan stream and print the file-level duplicates report",
 		Args:  cobra.MaximumNArgs(1),
-		Run: func(cmd *cobra.Command, args []string) {
+		Run: func(_ *cobra.Command, args []string) {
 			runReport(args)
 		},
 	}
@@ -61,21 +69,23 @@ func main() {
 		Use:   "trees [files.dat|-]",
 		Short: "Read a scan stream and print the duplicate-tree report",
 		Args:  cobra.MaximumNArgs(1),
-		Run: func(cmd *cobra.Command, args []string) {
+		Run: func(_ *cobra.Command, args []string) {
 			runTrees(args)
 		},
 	}
 
 	root.AddCommand(scanCmd, reportCmd, treesCmd)
-	if err := root.Execute(); err != nil {
+
+	err := root.Execute()
+	if err != nil {
 		// Cobra has already printed the error and usage to stderr;
 		// an invalid subcommand or bad arguments is a usage error.
-		os.Exit(2)
+		os.Exit(exitUsage)
 	}
 }
 
 // fatalf reports a fatal error and exits 1.
 func fatalf(format string, args ...any) {
 	fmt.Fprintf(os.Stderr, "sfdupes: "+format+"\n", args...)
-	os.Exit(1)
+	os.Exit(exitFatal)
 }