Add agent run supervisor worker

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>

pkg/agent/restore.go

@@ -60,9 +60,6 @@ func restoreCheckpoint(
 	runID string,
 	registry AgentRegistry,
 ) (*Result, error) {
-	if cp.Version != CheckpointVersion {
-		return nil, fmt.Errorf("cannot restore: unsupported checkpoint version %d", cp.Version)
-	}
 	emitHook(agent, func(h RunHooks) { h.OnRunRestore(ctx, agent, cp) })
 
 	switch cp.Status {