Programmer Humor

You seem to be fundamentally misunderstanding how the shared keys work, here. You don't get to choose which key you sign with, either private or public.

If you sign a statement, it uses your private key. Period. The methodology is only the original owner of the keys has access to the private key, so any signed statement also using that private key, is directly from the key owner. Alternatively, anyone has access to your private key, and can encrypt data using it, that only the owner, with access to the private key, can decrypt.

Anyone in the world can encrypt a message that only you can decrypt. That's the way it works. Alternatively, when you sign a statement, you're establishing that the owner of the key pair signed it, as only the owner has access to the private key. Then, anyone in the world can verify your signed statement using your public key.

You don't get to choose which one you use. In the example from my original post, the ssh-agent only needs reference to the location of one of the two keys, and will use the filename regardless, to distinguish between public and private. If I point the configuration option to this_is_my_key indicating a private key, it will automatically use this_is_my_key.pub as a public key. If I indicate this_is_my_key.pub then it does the reverse and will assume that this_is_my_key is the private key. It doesn't matter which one you specify, the ssh-agent just needs a filename to work with.