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/templates/config.yaml

@@ -105,6 +105,13 @@ monitoring:
         # Number of expired rows to delete per table batch
         delete_batch_size: 1000
 box:
+    # Master switch for the Box sandbox runtime. When false, LangBot does NOT
+    # attempt to connect to a remote Box runtime nor start a local stdio Box
+    # subprocess. Disabling Box also disables every feature that depends on it:
+    # the native sandbox tools (exec/read/write/edit/glob/grep), the activate
+    # skill tool, skill add/edit, and stdio-mode MCP servers. Skills can still
+    # be listed read-only and http/sse MCP servers continue to work.
+    enabled: true
     backend: 'local'  # 'local' (Docker/nsjail), 'docker', 'nsjail', or 'e2b'. BOX_BACKEND env var takes precedence.
     runtime:
         endpoint: ''  # External Box Runtime base URL, e.g. 'ws://127.0.0.1:5410'. Leave empty for local auto-managed runtime.