Rather than create a new SSH key pair on a vagrant box, I would like to re-use the key pair I have on my host machine, using agent forwarding. I've tried setting config.ssh.forward_agent to TRUE in the Vagrantfile, then rebooted the VM, and tried using:
vagrant ssh -- -A
...but I'm still getting prompted for a password when I try to do a git checkout. Any idea what I'm missing?
On Windows, the problem is that Vagrant doesn't know how to communicate with git-bash's ssh-agent. It does, however, know how to use PuTTY's Pageant. So, as long as Pageant is running and has loaded your SSH key, and as long as you've set config.ssh.forward_agent, this should work.
See this comment for details.
If you use Pageant, then the workaround of updating the Vagrantfile to copy SSH keys on Windows is no longer necessary.