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completeByDbId only cleaned up in-memory state, leaving sdk_sessions rows with status='active' and completed_at=NULL indefinitely. Ghost sessions accumulated and exhausted the agent pool, causing 60s timeout errors. - Add SessionStore.markSessionCompleted() to set status/completed_at/completed_at_epoch - Call it at the start of completeByDbId before in-memory cleanup - Inject SessionStore into SessionCompletionHandler via constructor - Add 4 tests covering status, timestamps, isolation, and non-existent IDs Closes #1532 Co-Authored-By: Claude <noreply@anthropic.com>