Previously completed sessions were suppressed for 5 minutes based on
JSONL file staleness. With fast-idle (cache-ttl detection), sessions
complete in ~3s — but the gateway immediately appends the next user
message, keeping the file 'fresh'. This blocked reactivation entirely.
Fix: compare sessions.json updatedAt against the completion timestamp.
If the gateway updated the session AFTER we marked it complete, a new
turn has started — reactivate immediately.
Pure infrastructure: timestamp comparison between two on-disk files.
No AI model state or memory involved.
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