Gate review: `degraded` became a whole-life serving state but four
sibling predicates only accepted `running`: derive_gateway_busy /
derive_gateway_drainable (the NAS drain gate reported a degraded gateway
with in-flight turns as idle and undrainable), the stale-heartbeat
detectors in `hermes gateway status` and the dashboard, and the Windows
doctor probe. All four accept `degraded`; a dead watchdog-stamped
`degraded` is still excluded by `gateway_running=False`.
The parked-platform ERROR pointed at `/platform resume`, which only
resumes platforms in the retry queue - a non-retryable failure never
enters it. The remedy is `hermes gateway restart`.
Test: a degraded gateway with active agents is busy; not live => not
drainable.
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