claudecode: count subagents launched by subagents

A nested subagent's launch is in its parent subagent's transcript, not the session's, so it was never found and its tokens were dropped. Follow launches through each subagent transcript; they share the session's subagents directory. Replaying 103 real local sessions across Claude, Codex, Copilot, Pi, Droid and Antigravity now matches an independent total for every one.