Relay backend: event-log fast-path in front of the roster-reconciliation exit watcher

[willwashburn]

The relay backend detects agent exits by reconciling tracked agents against the engine roster (15s poll) because the hosted engine emits no push event on exit. Once AgentWorkforce/relaycast#273 (durable agent.exited event) lands and cast.agentrelay.com runs an engine that carries it (AgentWorkforce/relaycast-cloud#33), add a fast-path:

• consume agent.exited (and node.status.offline) from the agent event stream / durable event log with a persisted cursor (FileStateStore)
• keep the reconciliation poll as the belt-and-braces detector at a longer interval
• carry the exit reason through so isCompletionReason gets task-exit fidelity instead of routing everything through the PR probe

Blocked on the two upstream issues above.

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