feat: xterm.js + node-pty architecture for Claude CLI web terminal

Complete rewrite from tmux + JSONL parsing to a clean PTY-based approach.
The proxy spawns Claude CLI in a pseudo-terminal via node-pty and relays
terminal I/O as binary WebSocket frames to the simple-chat backend,
which forwards them to the browser where xterm.js renders a full terminal.

- node-pty PTY manager with 50KB replay buffer per session
- Binary frame protocol with chatId multiplexing
- WebSocket client with auto-reconnection and heartbeat
- Directory listing and session listing for the web UI
- README with setup instructions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.env.example

@@ -0,0 +1,8 @@
+# Токен устройства (получить в simple-chat: Настройки → Устройство)
+PROXY_TOKEN=
+
+# WebSocket URL бэкенда simple-chat
+# Локальная разработка:
+SERVER_URL=ws://localhost:3000/ws/agent-proxy
+# Продакшен:
+# SERVER_URL=wss://ai-chat.vigdorov.ru/ws/agent-proxy