feat(ado-scheduler): attempt sessions once; disable retry-on-lease-expiry

Adds the CP1b ADO placement scheduler (previously untracked) with a stopgap
for the run-storm: SCHED_RETRY_ON_LEASE_EXPIRY (default off). When a root
session is still a candidate after its affinity lease lapses, decidePlacement
now returns skip-retry-disabled instead of re-queuing an ADO run. Each session
is attempted once until the CP2 fix lands (heartbeat-renewed short lease +
run-status reconcile + atomic dedup + TTL jitter). Set the flag to 1 to restore
blind retry-on-expiry.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 26f310eb-4680-4102-a8aa-f8d041940ab4

Author: andrewkcchung