feat(hook): trace Task subagents into the same Langfuse session

[LeonardoDaviti]

Subagents spawned via the Task tool run in isolated sub-sessions with their own transcripts under /subagents/agent-*.jsonl. The hook only parsed the parent transcript, so all subagent model calls, tool use and cost were dropped — only the orchestrator model appeared and session cost was undercounted.

Sweep the subagents/ directory on Stop/SessionEnd and feed each subagent transcript through the existing incremental pipeline (read_new_jsonl -> build_turns -> emit_turn), emitting it as a separate trace in the same session, linked to the spawning Task tool_use via parent_tool_use_id metadata and subagent / agent:<type> / parent_tool:<id> tags. Idempotent via per-file offset state; recurses for nested subagents (spawnDepth > 1).

emit_turn gains optional trace_name/root_name/extra_tags/extra_metadata so subagent traces reuse the same emission logic. main() no longer early-returns when the parent transcript has no new rows, so subagents are still swept.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}