Subagent sessions were missing tool call displays because the gateway's
passive subscribe mode (agent.subscribe unsupported) only broadcasts
assistant + lifecycle events, not tool events.
Three fixes:
- Use start-mode GatewayProcessHandle for subagent follow-up messages
so the agent RPC streams all events (including tools) on the same
WebSocket connection
- Enrich persisted subagent JSONL at load time from the gateway's
on-disk session transcript when tool-invocation parts are missing
- Best-effort enrichment at finalization time from gateway transcript
for subscribe-only runs that didn't receive tool events
045b73f42e