Use mcdsl/terminal.ReadPassword for secure password input

Replaces raw bufio.Scanner password reading (which echoed to terminal)
with the new mcdsl terminal package that suppresses echo via x/term.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/golang.org/x/sys/plan9/mksysnum_plan9.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+# Copyright 2009 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+COMMAND="mksysnum_plan9.sh $@"
+
+cat <<EOF
+// $COMMAND
+// MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
+
+package plan9
+
+const(
+EOF
+
+SP='[ 	]' # space or tab
+sed "s/^#define${SP}\\([A-Z0-9_][A-Z0-9_]*\\)${SP}${SP}*\\([0-9][0-9]*\\)/SYS_\\1=\\2/g" \
+	< $1 | grep -v SYS__
+
+cat <<EOF
+)
+EOF