Three surfaces start updates against one checkout: a terminal
"hermes update", the dashboard's Update button (which spawns that same
command detached), and the desktop's, which hands off to the Tauri
updater. Only the Tauri updater published the in-progress marker, and
only Electron read it -- to gate backend startup, not to stop a second
updater. So a dashboard-spawned update and an installer-driven git
checkout could mutate the same tree concurrently, rewriting source under
a live interpreter.
Claim the same marker from cmd_update rather than adding a second
mechanism: same path, same pid+started_at payload the Rust and Electron
readers already parse. A marker only counts as live when its pid is alive
and it is inside the shared age ceiling, so a crashed updater self-heals
instead of wedging every future update. Release only removes a marker we
still own, leaving a handoff partner's claim intact.
Refusing exits 2, matching the existing concurrent-instance contract the
Tauri updater already recognizes.
