Prototype native Windows SSH-agent relay for Docker Desktop and Podman

[BrettKinny]

Context

Windows OpenSSH exposes agent access through \\.\pipe\openssh-ssh-agent; Linux Boxes require a Unix-domain socket. Docker Desktop and Podman machine do not currently document one common supported bridge. Community solutions use WSL-specific npiperelay/socat relays.

Acceptance criteria

• Prototype without copying private keys or mounting the SSH directory read-write.
• Qualify Docker Desktop and Podman separately on supported Windows configurations.
• Document relay trust, lifetime, cleanup, stale-socket handling, and failure behavior.
• Pin and verify any third-party relay artifact before installation.
• Keep forwarding opt-in until both runtime paths are qualified.
• Add real Windows UAT and automated contract coverage where feasible.

Relationship

Split from #107 so the supported v1.3 cross-adapter migration contract is not coupled to an undocumented runtime transport.