[Feature]: Add Windows SSH agent support

[andya1lan]

Feature description

As a Windows user, I want Wave’s SSH connections to reuse my local SSH agent.
Today, agent detection only works via SSH_AUTH_SOCK (Unix-style), so Windows users can’t leverage the built-in OpenSSH agent.

Implementation Suggestion

1. Tries SSH_AUTH_SOCK first when set.
2. Otherwise dials the default Windows OpenSSH agent named pipe (//./pipe/openssh-ssh-agent).
3. Leaves non-Windows behavior unchanged.

Anything else?

No response