Delete subagent-panel.tsx (~670 lines) and its test. The SubagentPanel
bypassed the AI SDK's useChat pipeline with a manual createStreamParser,
causing tool events to never render. Instead, add a lightweight subagent
mode to ChatPanel via sessionKey/subagentTask/onBack props that reuses
the same DefaultChatTransport + useChat pipeline, fixing tool event
rendering and persisted message loading for completed subagent sessions.