Reject scan --workers below 1 as a usage error (closes #10)

A --workers value of 0 or less used to be quietly raised to 1, so a
typo ran the whole scan on one worker with nothing on stderr to say
why. scan now refuses it before anything is scanned: one line on
stderr and exit 2, like the other usage errors. The clamp in runScan
is gone, and README states the rule and the default.

Model: opus-5-5

main.go

@@ -51,6 +51,10 @@ const (
 // cobra prints for it is the whole message.
 var errNoSubcommand = errors.New("no subcommand")
 
+// errWorkersBelowOne is the usage error for a scan --workers value
+// below 1.
+var errWorkersBelowOne = errors.New("--workers must be at least 1")
+
 // Version is the build version, injected at link time via -ldflags
 // (see the Makefile); "dev" for a plain go build.
 //
@@ -98,8 +102,7 @@ func run(args []string, stdout, stderr io.Writer) int {
 
 		return exitFatal
 	default:
-		// A usage error: cobra has already printed the message and
-		// the usage text.
+		// A usage error, which cobra has already reported on stderr.
 		return exitUsage
 	}
 }
@@ -154,6 +157,18 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 		Use:   cmdScan + " [--workers N] [-x] PATH...",
 		Short: "Walk trees and synchronize the scan database",
 		Args:  cobra.MinimumNArgs(1),
+		PreRunE: func(cmd *cobra.Command, _ []string) error {
+			if scanWorkers < 1 {
+				// A usage error, reported in one line: cobra
+				// prints the message, and run exits 2.
+				cmd.SilenceUsage = true
+
+				return fmt.Errorf("%w, got %d", errWorkersBelowOne,
+					scanWorkers)
+			}
+
+			return nil
+		},
 		RunE: runE(func(ctx context.Context, args []string) error {
 			ctx, stop := interruptContext(ctx)
 			defer stop()