* feat(cli): guard history turns with --exclusive, --who and --if-open
* fix(cli): make exclusive history turns atomic
Re-read the open turn only after acquiring history ownership so
concurrent exclusive begins cannot both claim the same project.
Cover the race with a deterministic two-process regression test.
* fix(cli): refuse a second exclusive turn while a preview is open
The preview's begin window now rejects another agent's live turn, and an
idle turn no longer blocks exclusive begin. The two-process test waits on
pids collected from claim drafts instead of seeing both files at once.
Xuanru Li
7c73b3caee
