docs(agent): document cumulative-snapshot contract on CalculateTotalTokenUsage

Lift the cumulative-since-session-start subagent contract onto the SubagentAwareExtractor.CalculateTotalTokenUsage interface doc: SubagentTokens is cumulative (agent IDs from the full transcript, each subagent re-read from line 0), not a delta scoped to fromOffset, and callers must replace rather than sum and rescope windows via a baseline. A per-window-delta implementation would silently break the accounting. Shrink the claudecode/factoryaidroid NOTE blocks to one-line pointers at the interface contract, fixing their stale reference to SubagentTokensBaseline (it lives on session.State, not the strategy package).