Make the copyable recipe independent of ambient set -e behavior. A failed tmux new-session now reports the failure and exits while session_owned is still false; only a successful creation reaches the ownership assignment.
A real collision reproduced the prior snippet killing its sentinel after marking a failed creation as owned. The guarded version preserved the exact sentinel identity, and three refined fresh-agent runs again preserved IDs 17, 18, and 19 while cleaning only their fallback sessions.
Replace deterministic shared tmux names and preemptive deletion with a readable run-unique name, an ownership bit set only after successful creation, and cleanup guarded by that ownership. A collision now fails closed: the runner chooses another unique name or reports failure and never reclassifies the existing session as stale.\n\nwriting-skills pressure evidence used three fresh subjects per arm with real sentinel sessions. RED was 0/3: every subject followed the old recipe by killing the pre-existing session, recreating the same name, and cleaning it again. GREEN was 3/3: every subject preserved the original session identity, created a distinct owned session, captured READY, and removed only the owned session. The controller independently verified original IDs $9, $10, and $11 before removing the disposable sentinels.
