Feature: Add persistent remote terminal MCP tools

Add terminal_open / terminal_exec / terminal_close so an AI can hold one
shell session per Windows host and run a sequence of commands with cwd and
environment preserved, instead of the one-shot exec_command.

The persistent terminal is a separate full-command write capability gated by
McpTerminal (default off) plus McpReadonly=0, with no whitelist and full
audit. One device maps to one terminal session via the shared m_TermSessions
map; idle sessions are swept after 300s. terminal_exec rejects commands that
contain & or | (they corrupt the sentinel control-operator chain) as well as
control characters.

Also harden exec_command and terminal_exec against newline/CR injection, fix a
dangling subCtx after an abrupt shell disconnect, and refresh lastActiveAt on
command completion. Add en/zh-TW translations for the new UI strings and a
design document covering both exec_command and the persistent terminal.

Co-Authored-By: deepseek-v4-pro

server/2015Remote/lang/en_US.ini

@@ -1987,7 +1987,10 @@ MCP
 端口需为 1-65535 的数字=Port must be a number between 1 and 65535
 Token 不能为空=Token cannot be empty
 MCP 设置已保存。=MCP settings saved.
-启用/端口/绑定地址/Token/只读/白名单的改动需重启程序生效。=Changes to enable/port/bind/token/read-only/whitelist take effect after restart.
+启用/端口/绑定地址/Token/只读/白名单/持久终端的改动需重启程序生效。=Changes to enable/port/bind/token/read-only/whitelist/persistent terminal take effect after restart.
 只读模式（禁命令执行）=Read-only mode (disable command execution)
 命令白名单=Command whitelist
 MCP命令执行=MCP command execution
+启用持久终端（全命令，无白名单）=Enable persistent terminal (full commands, no whitelist)
+持久终端仅在只读模式关闭时生效。=Persistent terminal takes effect only when read-only mode is off.
+MCP持久终端=MCP persistent terminal