Support SSH agent authentication on Windows

**What do you want?**

Allow `k3sup install` and other SSH-based commands to use keys loaded in the Windows OpenSSH Authentication Agent, as they already do on Linux and macOS.

**Why do you want this?**

On Windows, k3sup currently skips SSH-agent authentication and falls back to the default private-key path. This fails when the key is available through the Windows SSH agent but no `~/.ssh/id_rsa` file exists.

Supporting the Windows agent would also avoid requiring users to hardcode a private-key path with `--ssh-key`.

**Recommended solution**

Use `github.com/Microsoft/go-winio` to connect to the standard Windows OpenSSH agent named pipe:

`\\.\pipe\openssh-ssh-agent`

Pass the resulting `net.Conn` to the existing `golang.org/x/crypto/ssh/agent` authentication flow. If the agent is unavailable or authentication fails, retain the existing private-key fallback.

**Additional context**

I implemented and tested this against the built-in Windows OpenSSH Authentication Agent. k3sup successfully authenticated to a remote Ubuntu server and installed K3s without `--ssh-key` or a default `~/.ssh/id_rsa` file.

While implementing this, I also found that the existing SSH-agent connection was not closed. The proposed change makes its lifecycle explicit: the connection is closed after a failed agent authentication attempt before falling back to a private key, and when the successful SSH operator is closed. A regression test covers the failure and fallback path.