Add standalone subagent invocations per session histogram

Broaden filter: drop non-conversational classifier calls (max_tokens<=64, no tools) — removes residual OSL 5-10 spike

Refresh corpus: 219 traces, Security Monitor subagent excluded at source (drops OSL 5-10 spike)

Initial upload: 175 traces with subagents, v5-only + ≥20 main turns + CC ≥ 2.1.139