createReportBundle's addSubagentSessions scanned path.dirname(sessionFile)
- the sessions root holding every session in history - and bundled the 10
newest .jsonl files, leaking unrelated sessions' transcripts and bloating
archives to tens of MB. Real subagent sessions live inside the current
session's own artifacts dir, so recurse that subtree and drop the
sessions-root scan entirely.
Fixes#8648
roboomp