docs(agent): note deliberate double transcript parse in subagent scan

CalculateTotalTokenUsage and ExtractAllModifiedFiles (Claude Code and Factory AI Droid) parse the transcript twice per call: the sliced window plus a full-transcript scan to find subagents spawned before the window (#329). Record why the redundant full parse is kept: ParseFromBytes drops malformed lines so a parsed-entry index does not map to a raw line number, making a safe single-pass split require threading raw-line numbers through the shared parser, and a Task-marker line scan would duplicate the nested tool_result decoding. The common no-subagent case already avoids the cost via the subagentsDir == "" short-circuit.