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/telemetry/telemetry.py

@@ -68,10 +68,21 @@ class TelemetryManager:
                     'edition',
                     'error',
                     'timestamp',
+                    'event_type',
                 ):
+                    if sfield not in sanitized:
+                        continue
                     v = sanitized.get(sfield)
                     sanitized[sfield] = '' if v is None else str(v)
 
+                # event_type defaults to 'query' for backward compatibility
+                if not sanitized.get('event_type'):
+                    sanitized['event_type'] = 'query'
+
+                # features must be a JSON object
+                if 'features' in sanitized and not isinstance(sanitized['features'], dict):
+                    sanitized['features'] = {}
+
                 if 'duration_ms' in sanitized:
                     try:
                         sanitized['duration_ms'] = (