five POST handlers (post_kill / post_restart / post_start / post_rebuild)
were all repeating the same boilerplate: strip prefix, set_transient,
call lifecycle::X, clear_transient, match the result. extract one
helper that takes the transient kind, error-message verb, the work
body, and an optional 'on success' tail (used by kill to also
unregister + emit HelperEvent::Killed). each handler shrinks to a
single lifecycle_action(..) call. zero behavior change.
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