chat-hook: no-op on /backlog kick if not synced
This matches the behavior that's used for +kick on /mailbox, among other flows.
::
?: ?=([%backlog @ *] wir)
=/ pax `path`(oust [(dec (lent t.wir)) 1] `(list @ta)`t.wir)
+ ?. (~(has by synced) pax) [~ state]
=/ mailbox=(unit mailbox) (chat-scry pax)
=. pax ?~(mailbox wir [%mailbox pax])
:_ state
