Turn boundaries now capture pi session-tree refs: each provider turn's
nativeTurnRef becomes its first user entry id and the thread's
nativeConversationHeadRef tracks the tree leaf, giving rollback a durable
target. rollbackThread re-roots the active branch with pi's fork command,
so reverting a checkpoint also rewinds the provider conversation instead
of marking it divergent. Thread titles sync into pi's session name so T3
threads stay identifiable in pi's own /resume listing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>