refactor(box): run Box Runtime as subprocess inside LangBot container

Remove the separate langbot_box_runtime Docker service. Box Runtime
  now always launches as a local stdio subprocess, regardless of whether
  LangBot runs in Docker or not. The WebSocket transport path is kept
  only for explicit runtime_url configuration (remote deployment).

  This simplifies deployment by eliminating cross-container path mapping
  and network hops. Box Runtime is a pure scheduling process (talks to
  Docker socket / nsjail), it does not execute user code or touch the
  filesystem, so container isolation is unnecessary — unlike Plugin
  Runtime.

src/langbot/templates/config.yaml

@@ -89,7 +89,7 @@ monitoring:
         check_interval_hours: 1
 box:
     profile: 'default'
-    runtime_url: ''  # Leave empty to use defaults: http://127.0.0.1:5410 locally, http://langbot_box_runtime:5410 in Docker
+    runtime_url: ''  # Leave empty to auto-launch local Box Runtime subprocess. Set to a URL only for remote/external deployment.
     shared_host_root: './data/box'  # For Docker deployment, use '/workspaces'
     default_host_workspace: ''  # Defaults to '<shared_host_root>/default'
     allowed_host_mount_roots:  # Defaults to ['<shared_host_root>'] when left empty