agent forwarding: the redial re-offer and the reverse pin are asserted below the wire

open   by a73x

Two behaviours the design leans on, neither watched end to end.

**The redial re-offer.** A redial is a fresh attach onto a fresh daemon-side
slot that remembers no offer, so `sendAttach` re-sends `agent_offer` every
time. Structural — one funnel, so it cannot be forgotten — but no e2e leg
kills a link mid-session and asserts forwarding still answers afterwards.

**The reverse pin.** A channel already open stays with the client that
opened it even when activity flips to another `-A` client; only the NEXT
dial follows the new leader. Unit tests assert it. End to end it needs a
signing operation slow enough to flip activity underneath it, which is why
it was never written.

Both are the kind of thing that stays true until someone refactors the
attach path.