agent forwarding: mux -A HOST (ssh, then the QUIC handoff) has no e2e leg

open   by a73x

Every agent e2e leg attaches over `--sock` — a local daemon, one process
hop. The transport people will actually type is `mux -A HOST`: ssh to the
box, then a handoff onto QUIC mid-session.

Nothing proves the offer survives that handoff. `sendAttach` re-offers on
every attach and a handoff is a fresh attach, so it should — but "should"
is the word this repo does not accept. The field trial that shipped
v0.0.1-12-pre2 exercised it by hand and it worked; that is not a gate.

Wants: an e2e leg that forwards over ssh and asserts `ssh-add -l` still
answers after the QUIC handoff has happened. The rig already knows how to
stand up a QUIC daemon; what it lacks is a leg that changes transport
underneath a live agent channel.