feat(windows): decide ssh-agent forwarding support (unsupported, warning-only, or named-pipe bridge) #659

🤖: AI agent (OpenCode), filing on behalf of @mogul. Split out of the epic #461 (Increment 2) so it is tracked on its own.

Problem

Host ssh-agent forwarding assumes Unix sockets. Windows OpenSSH and Pageant use services or named pipes. The Windows preview path skips or warns on forwarding, but no decision has been recorded for the supported path.

Decision needed

Choose one: (a) unsupported, (b) warning-only, or (c) implemented via a named-pipe bridge. Record it as an ADR (docs/adr/) and document the user-visible behavior.

Related

#461, #501 (SSH signing and vsock route), #402.