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/core/app.py

@@ -200,6 +200,17 @@ class Application:
                 scopes=[core_entities.LifecycleControlScope.APPLICATION],
             )
 
+            # Telemetry instance heartbeat (startup + daily); respects
+            # space.disable_telemetry via TelemetryManager.send().
+            if self.telemetry is not None:
+                from ..telemetry import heartbeat as telemetry_heartbeat
+
+                self.task_mgr.create_task(
+                    telemetry_heartbeat.heartbeat_loop(self),
+                    name='telemetry-heartbeat',
+                    scopes=[core_entities.LifecycleControlScope.APPLICATION],
+                )
+
             # Start monitoring data cleanup task if enabled
             monitoring_cfg = self.instance_config.data.get('monitoring', {})
             auto_cleanup_cfg = monitoring_cfg.get('auto_cleanup', {})