Will it slowly ruin the battery to leave a MacBook Pro (Retina, 15-inch, Early 2013) plugged in at 100% all the time?
This questions risks inviting a lot of opinion, so some may vote to close this down. 

However, to try and keep this to the facts, I refer you firstly to what Apple has to say about their batteries. 

In a nutshell, the key indisputable points are:


Apple lithium-ion batteries use fast charging to reach 80% of their capacity and then switch to slower trickle charging to complete the charge.
Charge your MacBook Pro whenever you want. Many people believe you need to let them discharge 100% before recharging. This is not true.
Ambient temperature is one of the biggest factors in battery health/life. MacBooks are designed to work at their best when ambient temperatures are between 16° to 22° C (62° to 72° F).
It’s important to avoid exposing your MacBook to ambient temperatures higher than 35° C (95° F)
If you're ever going to store your MacBook for long-term without usage, it is best to store it totally powered down and at only about half charge.


Also, the latest research confirms one of the most important determinants of what ruins a battery is time. (You could purchase a brand new MacBook Pro battery and leave it unused in its original packaging for five years and then find it's capacity is nowhere near what it should be when you start using it) 

Finally, since you're using your MacBook Pro in clamshell mode, you may find this general info useful.