Filter to disable for certain custom post types

Something like:

```
static function enabled() {
return (bool) apply_filters( 'hidden_posts_enabled', true );
}
```

Checks for `enabled()` in:
- `pre_get_posts()`
- `hidden_checkbox()`
- `save_meta()`

Locate the plugin code defining enabled(), then inspect its checks in pre_get_posts(), hidden_checkbox(), and save_meta(). Add a hidden_posts_enabled filter that can be disabled for selected custom post types, and verify that all three paths honor the disabled result.

Written by the indexing model from the issue text.