Copied fork messages keep their parent seqs, but a fresh checkpoint's
baseline_seq starts near zero, so nothing folded: forks received a new
full baseline plus every stale context-update message from the parent,
and copied compaction rows triggered a spurious rebaseline. Inheriting
the parent row keeps the fold horizon consistent with the copied
transcript.
Also asserts the checkpoint reset on committed reverts and deletes the
unused SessionStore.runnerContext / SessionHistory.loadForRunner
surface.
Kit Langton
aa2c1472fa