fix: enable shell mode for ACP agent spawn on Windows On Windows, `npx` is a `.cmd` batch wrapper. Node's `spawn()` doesn't resolve `.cmd` extensions, causing ENOENT even when npx is on PATH. Adding `shell: true` on win32 lets cmd.exe handle the resolution. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>