Two related fixes to what a btw session inherits and shows.
**Fork point.** `/btw` is typically typed *while* the main thread is
working — that is the moment a side question comes up. Forking at HEAD
then clones a turn that is still streaming, so the fork inherits a
truncated assistant message and the user instruction that provoked it as
the newest, most salient thing in its context. Fork at the parent's last
completed assistant turn instead, read from the sync store with no extra
round-trip. A parent with no completed turn yet keeps the previous
fork-at-HEAD behavior.
**Boundary.** `filterBtwTailMessages` shows every inherited message when
the boundary is `null`, and the boundary is `null` whenever the
newest-cloned read comes back empty. That is correct for a fork of an
empty parent, but it also turns an empty read into "nothing was
inherited" for a fork that demonstrably did inherit history — the panel
then opens with the parent's whole transcript in it. Having picked a
fork point proves the parent had turns, so fall back to that id instead
of `null`. The fork's own messages are created later and still sort
after it, so its tail stays complete either way.
dibanez
b0083f6e0f