feat: add dev mode with DEV_ env prefix

Support dual configuration: PROXY_TOKEN/SERVER_URL for production,
DEV_PROXY_TOKEN/DEV_SERVER_URL for local development.

- npm start → production
- npm run start:dev → dev (PROXY_MODE=dev)
- npm run dev → dev with hot-reload
- Restore PTY auto-creation on sync_state for existing chats

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

package.json

@@ -5,7 +5,8 @@
   "type": "module",
   "scripts": {
     "start": "tsx src/main.ts",
-    "dev": "tsx watch src/main.ts",
+    "start:dev": "PROXY_MODE=dev tsx src/main.ts",
+    "dev": "PROXY_MODE=dev tsx watch src/main.ts",
     "lint": "eslint src/",
     "typecheck": "tsc --noEmit"
   },