feat(box): add --standalone-box flag and 3-way transport decision for Box runtime

Align Box runtime connection logic with Plugin runtime's pattern:
- Docker: WebSocket to langbot_box container (ws://langbot_box:5411)
- --standalone-box: WebSocket to external Box process (ws://localhost:5411)
- Windows: subprocess + WebSocket (workaround for async stdio limitation)
- Unix/macOS: subprocess + stdio pipe (unchanged)

BoxRuntimeConnector now inherits ManagedRuntimeConnector for subprocess
lifecycle reuse. Add langbot_box service to docker-compose.yaml.

src/langbot/__main__.py

@@ -29,6 +29,12 @@ async def main_entry(loop: asyncio.AbstractEventLoop):
         help='Use standalone plugin runtime / 使用独立插件运行时',
         default=False,
     )
+    parser.add_argument(
+        '--standalone-box',
+        action='store_true',
+        help='Use standalone box runtime / 使用独立 Box 运行时',
+        default=False,
+    )
     parser.add_argument('--debug', action='store_true', help='Debug mode / 调试模式', default=False)
     args = parser.parse_args()
 
@@ -37,6 +43,11 @@ async def main_entry(loop: asyncio.AbstractEventLoop):
 
         platform.standalone_runtime = True
 
+    if args.standalone_box:
+        from langbot.pkg.utils import platform
+
+        platform.standalone_box = True
+
     if args.debug:
         from langbot.pkg.utils import constants