fix(agentimport): keep subagent baseline when a turn's snapshot is nil

rescopeSubagentTokensToDeltas advanced prevCumulative unconditionally, even when a turn's cumulative SubagentTokens was nil (the subagent transcript transiently failed to read, so CalculateTotalTokenUsage continue-d past it). A later turn whose snapshot reappeared then subtracted a nil baseline and re-reported the full cumulative, reintroducing the double-counting the rescoping removes. Only advance prevCumulative when the snapshot is non-nil, matching the live path where accumulateTokenUsage replaces SubagentTokens only for non-nil snapshots.