docs(session): record subagent "latest snapshot wins" tradeoff

Document that TokenUsage.SubagentTokens is replaced (not summed): a mid-session main-transcript reset or rotation can make a later cumulative snapshot smaller than an earlier one, so the session-wide subagent total regresses (undercounts) for the rest of the session. This is accepted over the prior multiplicative overcount, and checkpoint deltas are unaffected because CheckpointTokenUsage.SubagentTokens is (cumulative - baseline) floored at zero by clampSubtract.