getgrav/grav ? reverse-engineered prompt

Build me a fast and flexible content management system using PHP. The most important feature is that it should be a "flat-file" CMS—I don't want to mess with setting up a database. All the website content, like pages and blog posts, should just be simple Markdown files organized in a folder structure.

It should use Twig for templating so it's easy to create and modify themes, and simple YAML files for all the configuration. It also needs a powerful package manager I can use from the command line to easily find and install new plugins and themes. This will let me add new functionality and change the look of the site easily. Performance is a big deal, so make sure there's a good caching layer built-in. You can use modern, best-in-class PHP libraries like Symfony components to build it out.