* feat: add assistant turn branching
* fix(threads): skip workspace clone when branching from historical turn
Workspace files are not checkpointed, so cloning them onto a branch
rooted at an older assistant turn leaked files created in a later
timeline. Restrict the best-effort workspace copy to branches taken from
the latest turn; historical-turn branches now report
workspace_clone_mode="skipped_historical_turn" and keep only the
restored message history.
* style(frontend): fix prettier formatting in e2e mock-api
Collapse the branch-title normalization chain onto a single line to
satisfy the frontend lint (prettier --check) CI gate.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
