Add HandStarted, HandCompleted, and HandFailed event variants to
ObserverEvent, and HandRunDuration, HandFindingsCount, HandSuccessRate
metric variants to ObserverMetric. Update all observer backends (log,
noop, verbose, prometheus, otel) to handle the new variants with
appropriate instrumentation. Prometheus backend registers hand_runs
counter, hand_duration histogram, and hand_findings counter. OTel
backend creates spans and records metrics for hand runs.
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