feat(box): support custom sandbox container image via config.yaml

Add 'image' field to box config section. When set, it overrides the
profile default image (python:3.11-slim) for all sandbox containers.
Priority: caller-specified > config.yaml image > profile default.

src/langbot/pkg/box/service.py

@@ -60,6 +60,7 @@ class BoxService:
         self.allowed_host_mount_roots = self._load_allowed_host_mount_roots()
         self.default_host_workspace = self._load_default_host_workspace()
         self.profile = self._load_profile()
+        self.custom_image = self._load_custom_image()
         self.workspace_quota_mb = self._load_workspace_quota_mb()
         self._recent_errors: collections.deque[dict] = collections.deque(maxlen=_MAX_RECENT_ERRORS)
         self._shutdown_task = None
@@ -209,6 +210,10 @@ class BoxService:
         if spec_payload.get('workspace_quota_mb') in (None, '') and self.workspace_quota_mb is not None:
             spec_payload['workspace_quota_mb'] = self.workspace_quota_mb
 
+        # Global custom image overrides profile default (but not caller-specified image)
+        if self.custom_image and 'image' not in spec_payload:
+            spec_payload['image'] = self.custom_image
+
         self._apply_profile(spec_payload)
 
         try:
@@ -360,6 +365,10 @@ class BoxService:
             default_host_workspace = os.path.join(self.shared_host_root, 'default')
         return os.path.realpath(os.path.abspath(default_host_workspace))
 
+    def _load_custom_image(self) -> str | None:
+        raw = str(_get_box_config(self.ap).get('image', '') or '').strip()
+        return raw or None
+
     def _load_workspace_quota_mb(self) -> int | None:
         raw_value = _get_box_config(self.ap).get('workspace_quota_mb')
         if raw_value in (None, ''):

src/langbot/templates/config.yaml

@@ -89,6 +89,7 @@ monitoring:
         check_interval_hours: 1
 box:
     profile: 'default'
+    image: ''  # Custom sandbox container image. Leave empty to use the profile default (python:3.11-slim).
     runtime_url: ''  # Action-RPC WebSocket URL of an external Box Runtime. Leave empty for auto-detection (stdio locally, Docker service in containers).
     shared_host_root: './data/box'  # For Docker deployment, use '/workspaces'
     default_host_workspace: ''  # Defaults to '<shared_host_root>/default'