Poll-based supervisor that claims PENDING agent runs with FOR UPDATE
SKIP LOCKED, runs them with lease-based heartbeat, and handles
graceful shutdown. On infrastructure stop the row stays RUNNING so
stale recovery resets it to PENDING on restart; Restore picks up
from the last checkpoint. Heartbeat loss cancels execution without
committing a terminal status.
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>
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