After a dashboard restart the only record of which update THIS dashboard
started was the `=== hermes-update started ... <id> ===` header, read from a
2,000-line tail. A build that printed more than 2,000 lines (or a rotated
log) hid it: spawned_action_id became None, the matching root completion
marker and the matching archived success receipt were both refused, and the
route answered running=false/exit_code=null, which the web poller shows as
"Action failed (exit ?)".
_spawn_hermes_action now also writes the id to a `<name>.action_id` sidecar
(atomic) next to the log; the status route falls back to it when no header
is visible. A visible header still wins (a dashboard predating the sidecar
wrote only the header, and the newest header is the latest spawn). The
identity selects B's own archived receipt even when another profile's run is
latest; failed/unfinished actions still report exit null.
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