Port of #96340 (Finn763) onto the current legacy-scoped retry gates,
narrowed to the watchdog sentinel only: exit 124 + the "Update complete!"
marker maps to success so the hand-off verifies and relaunches the new
build instead of re-running the completed update (the 70+ minute popup
retry storm). A plain exit 1 after the marker is the stale-gateway
verdict (hermes_cli/update_receipt.py) and must keep failing.
As first written this also added a posix.sh gate and a retry-state
helper with a -SelfTestRetryState arm; the next commit drops the posix
gate (nothing there exits 124) and replaces the helper and self-test
with one end-to-end test through the real hand-off.
Co-authored-by: Finn763 <165816600+Finn763@users.noreply.github.com>
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