Unable to manage robots.txt in Craft
You can simply create a 'robots.txt' file in your templates folder and populate it using twig (templates do not need to end in .twig). Not sure what you're trying to control exactly, but something like this should technically work.

User-agent: *
{% set entries = craft.entries.limit(null) %} 
{% for entry in entries %}
    Disallow: {{ entry.uri }}
{% endfor %}


Terrible example but you get the idea. Then just delete the robots.txt file that is in your public folder.