feat(telemetry): payload v2 with feature usage counters and instance heartbeat

Per-query events now carry event_type='query' and a features JSON object:
- tool_calls by source (native/plugin/mcp/skill) via ToolManager
- tool_call_rounds, kb usage (count/engine plugins/retrieved entries) via local-agent
- sandbox execs/errors via BoxService
- activated_skills and bound mcp_servers snapshots

New instance_heartbeat event (startup + daily) reports anonymous instance
profile: deploy platform, database/vdb kind, box backend/availability,
adapter type names, and resource counts. Respects space.disable_telemetry.

All collection helpers are defensive and never break the pipeline.
Verified: ruff, 37 telemetry unit tests (13 new), 504 box/provider/pipeline tests.

src/langbot/pkg/box/service.py

@@ -12,6 +12,7 @@ import pydantic
 
 from langbot_plugin.box.client import BoxRuntimeClient
 from .connector import BoxRuntimeConnector, _get_box_config
+from ..telemetry import features as telemetry_features
 from langbot_plugin.box.errors import BoxError, BoxValidationError
 from langbot_plugin.box.models import (
     BUILTIN_PROFILES,
@@ -218,6 +219,7 @@ class BoxService:
             f'query_id={query.query_id} '
             f'summary={json.dumps(self._summarize_result(result), ensure_ascii=False)}'
         )
+        telemetry_features.increment(query, 'sandbox', 'execs')
         return self._serialize_result(result)
 
     def resolve_box_session_id(self, query: pipeline_query.Query) -> str:
@@ -785,6 +787,7 @@ class BoxService:
     # ── Observability ─────────────────────────────────────────────────
 
     def _record_error(self, exc: Exception, query: pipeline_query.Query):
+        telemetry_features.increment(query, 'sandbox', 'errors')
         self._recent_errors.append(
             {
                 'timestamp': _dt.datetime.now(_UTC).isoformat(),