feat(box): add box.enabled toggle and gate consumers on availability

Make the Box sandbox runtime optional. When ``box.enabled`` is false in
config (or when an enabled Box fails to connect), every dependent feature
degrades to the same disabled-state UX rather than crashing or silently
falling back to less safe code paths.

Backend:

- config.yaml: new top-level ``box.enabled: true`` flag (default true)
- BoxService:
  - Read box.enabled on construction
  - initialize() short-circuits when disabled — no remote WS connect, no
    stdio subprocess fork
  - _on_runtime_disconnect is a no-op when disabled (no reconnect loop
    on a deliberately-off service)
  - get_status() now exposes ``enabled`` so the frontend can tell
    "disabled in config" from "configured but failed"
- MCP stdio loader (mcp_stdio.uses_box_stdio): requires box_service to
  be available, not just installed
- MCP _init_stdio_python_server: when ap.box_service exists but is
  unavailable, refuse the stdio server with an actionable error instead
  of silently falling through to host-stdio (which bypasses the sandbox
  the operator asked for). Setups without ap.box_service installed at
  all keep the legacy host-stdio fallback for pre-Box dev mode
- SkillService._require_box_for_write: refuses create/update/install/
  write_skill_file when ap.box_service is installed but unavailable.
  Distinguishes disabled vs failed in the error message so the UI can
  surface the right hint. Legacy setups (no ap.box_service) keep the
  local fallback path — that distinction is what keeps the existing
  local-skills tests valid

Tests:
- Box disabled-state behavior (4 cases)
- Skill write refusal in disabled & failed states (7 cases)
- MCP stdio runtime info policy updated to match new refuse-when-down
  behavior

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/langbot/pkg/provider/tools/loaders/mcp_stdio.py

@@ -105,7 +105,14 @@ class BoxStdioSessionRuntime:
     def uses_box_stdio(self) -> bool:
         if self.server_config.get('mode') != 'stdio':
             return False
-        return getattr(self.ap, 'box_service', None) is not None
+        box_service = getattr(self.ap, 'box_service', None)
+        if box_service is None:
+            return False
+        # When Box is configured but currently unavailable (disabled or
+        # connection failed), do NOT silently fall through to host-stdio —
+        # that would bypass the sandbox the operator asked for. The caller
+        # is expected to refuse the stdio MCP server with a clear error.
+        return bool(getattr(box_service, 'available', False))
 
     async def initialize(self) -> None:
         await self._wait_for_box_runtime()