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>

tests/unit_tests/provider/test_mcp_box_integration.py

@@ -561,7 +561,12 @@ class TestGetRuntimeInfoDict:
         assert info['box_session_id'] == 'mcp-shared'
         assert info['box_enabled'] is True
 
-    def test_stdio_session_waits_for_unavailable_box_runtime(self, mcp_module):
+    def test_stdio_session_refuses_when_box_unavailable(self, mcp_module):
+        """Policy: when Box is configured but unavailable (disabled in config
+        OR connection failed), stdio MCP servers are NOT treated as box-stdio.
+        ``_init_stdio_python_server`` will raise a clear refusal at start
+        time; until then, the runtime info simply omits box_session_id so the
+        UI can render the disabled state cleanly."""
         ap = _make_ap()
         ap.box_service.available = False
         s = _make_session(
@@ -576,8 +581,8 @@ class TestGetRuntimeInfoDict:
             ap=ap,
         )
         info = s.get_runtime_info_dict()
-        assert info['box_session_id'] == 'mcp-shared'
-        assert info['box_enabled'] is True
+        assert 'box_session_id' not in info
+        assert 'box_enabled' not in info
 
     def test_stdio_session_without_box_service_uses_local_stdio(self, mcp_module):
         ap = _make_ap()