- Close WebSocket when session exits (exit event listener) to prevent
orphaned listeners and stale writes to dead PTY
- Add readyState guard in onTerminal to stop buffering after socket closes
- Simplify heartbeat: remove redundant alive flag, use pongTimeout only
- Add exponential backoff reconnection on unexpected WS close (skip for
server rejections 4004/4008/4009)
- Clear CJK textarea on session switch to prevent wrong-session input
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>