cleanup: remove Linux scripts, unify core dir, fix Intel Mac bug

- Delete usb-scripts/, Linux-*.sh, setup-linux-usb.sh, migrate.js
- Rename core-mac/core-win → core (with migration shims for existing USBs)
- Fix Mac-Menu.command ARM64 hardcode — now detects Intel/Apple Silicon
- setup.sh: remove Linux branch, Windows download only with --all-platforms
- Update README.md and CLAUDE.md to reflect Mac + Windows only

portable/Windows-Menu.bat

@@ -3,7 +3,12 @@ chcp 65001 >nul 2>&1
 title U-Claw Menu
 
 set "UCLAW_DIR=%~dp0"
-set "CORE_DIR=%UCLAW_DIR%app\core-win"
+set "APP_DIR=%UCLAW_DIR%app"
+
+REM Migration shim: rename old core-win to core for existing USB users
+if exist "%APP_DIR%\core-win" if not exist "%APP_DIR%\core" ren "%APP_DIR%\core-win" core
+
+set "CORE_DIR=%APP_DIR%\core"
 set "DATA_DIR=%UCLAW_DIR%data"
 set "STATE_DIR=%DATA_DIR%\.openclaw"
 set "NODE_DIR=%UCLAW_DIR%app\runtime\node-win-x64"