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

CLAUDE.md

@@ -88,7 +88,6 @@ Release artifacts go to GitHub Releases, not the repo.
 ## Platform Support Status
 
 - Mac Apple Silicon (ARM64): ✅ Working
-- Windows x64: 🚧 In development
-- Linux x86_64: ✅ Working (scripts + live USB mode)
-- Linux ARM64: ✅ Working (scripts)
 - Mac Intel (x64): ✅ Working（portable 需先运行 setup.sh 下载 node-mac-x64）
+- Windows x64: 🚧 In development
+- Linux: 不提供脚本，后续只提供教程