fix(attribution): scan full transcript for subagents spawned before checkpoint

ExtractAllModifiedFiles and CalculateTotalTokenUsage extracted spawned subagent IDs from the startLine-sliced transcript, so a subagent spawned before the checkpoint's startLine was missed once startLine advanced — its later file modifications and token usage went uncounted. Extract agent IDs from the full transcript (startLine=0) while keeping main-agent extraction scoped to the slice. Covers both the Claude Code and Factory Droid paths.

Closes #329