This is related to the "Open file" support in GitHub for Mac.
However, the fact that anything was added to the keychain is a bug that's since been fixed. You can safely delete the certificate from your keychain and it won't be re-added.
Question
I have a certificate in my Keychain called "ghconduit.com" and I have no clue what it is used for.
According to the WHOIS of the domain it belongs to GitHub but I can't find any information about its purpose.
La solution
This is related to the "Open file" support in GitHub for Mac.
However, the fact that anything was added to the keychain is a bug that's since been fixed. You can safely delete the certificate from your keychain and it won't be re-added.