write a pane's options in one tmux command list

the watcher wrote @agent_state, then @dingger_hook, as separate tmux
calls. a daemon stopped between them left a done display state next to a
stale "working" hook state, which the restarted daemon trusted: the agent
came back as working. now every changed option of a pane is one command
list, which tmux applies as a unit (and one process instead of several).

tests/kill-race.sh kills the daemon the instant an agent shows done: 24 of
40 kills left the options inconsistent before, 0 of 40 now. e2e step 17
runs it 20 times; the previous binary fails it.

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