Plugins can hand the host a file name, MIME type and bytes through the new
`save_file` import (plugin API 4, `permissions.saveFile`). The host asks the
user where to save (browser download on the web), so the plugin never sees a
path. Includes the SDK wrapper and the `export-names` example plugin.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Assisted-by: Claude (Anthropic)
