By my admittedly limited understanding of how HTTPS/TLS works, the end user (me) initiates a connection with a remote server which signs every one of its messages with a public key. This public key can be verified (magically) by checking the certificate, which is signed by a CA that vouches for the integrity of that certificate.

The upshot of this is that if I trust a CA, that CA can sign any certificate and say it is valid and my machine will be just fine with it; if a rogue CA is added to the trusted registry of my computer, then anyone who knows that rogue CA will be able to get their cert signed and pose as - potentially - any website and perform a man in the middle attack.

My corporation has just added their own cert as a root CA to all computers in the network. Should I therefore assume that all traffic I send is compromised?

Yup.
Yes. (If you consider "My company's admins can change my HTTP(s) traffic" compromise.) Except for some programs that pin their certificate use and fail if another certificate is used.
That's pretty much the idea of SSL inspection: Open up SSL/TLS, do some anti-virus-scanning, close up SSL/TLS again.
And in place of that "do some anti-virus-scanning" you can substitute any manipulation of the traffic, you want.
But on the other hand: whoever can install a root CA on your system has superuser power already. So there's a million other ways that they can interfere/listen in.
