Source live task and subagent events from the Remote Control read stream #390

The tracker keeps the background tasks and subagents a Remote Control session reports, from the worker's event batches and from the client read stream, as a bounded per-session book (applyAgentTaskEvent in src/agents/liveTasks.ts, filed by createRcSessionTracker). Shapes and join keys are in docs/rc-agent-events.md. Landed with the merge in #388, which closed #364; this issue tracks the source separately from the merge.