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_test.go

@@ -295,6 +295,42 @@ func TestRunUsageErrors(t *testing.T) {
 	}
 }
 
+func TestRunScanRejectsWorkersBelowOne(t *testing.T) {
+	// README §scan mode: --workers below 1 is a usage error reported in
+	// one line on stderr, before the scan opens the database.
+	for _, workers := range []string{"0", "-1"} {
+		t.Run(workers, func(t *testing.T) {
+			dbPath := testDBPath(t)
+			t.Setenv(databaseEnv, dbPath)
+
+			var stdout, stderr bytes.Buffer
+
+			args := []string{cmdScan, "--workers", workers, t.TempDir()}
+
+			code := run(args, &stdout, &stderr)
+			if code != exitUsage {
+				t.Errorf("run(%v) = %d, want %d", args, code, exitUsage)
+			}
+
+			want := "Error: --workers must be at least 1, got " + workers +
+				"\n"
+			if got := stderr.String(); got != want {
+				t.Errorf("stderr = %q, want %q", got, want)
+			}
+
+			if got := stdout.String(); got != "" {
+				t.Errorf("stdout = %q, want nothing (data only)", got)
+			}
+
+			_, err := os.Stat(dbPath)
+			if !errors.Is(err, fs.ErrNotExist) {
+				t.Errorf("stat %s: %v, want the database never created",
+					dbPath, err)
+			}
+		})
+	}
+}
+
 func TestRunHelp(t *testing.T) {
 	t.Parallel()