fix: debug reasoning execution flow and fix JSON serialization

- Add debug log for execute_immediately, violations, steps_count
- Fix JSON serialization in _store_session using default=str
- Plan object might contain non-serializable types (e.g. datetime)

harnessed_reasoning/core.py

@@ -470,6 +470,7 @@ class HermesReasoningEngine:
             })
 
             # Step 5: Execute if requested and safe
+            info(f"[reasoning] execute_immediately={execute_immediately}, violations={violations}, steps_count={len(plan.get('steps', []))}")
             if execute_immediately and not violations and plan.get('steps'):
                 exec_results = await self._execute_plan(
                     session_id, plan['steps'], config, context
@@ -602,12 +603,14 @@ class HermesReasoningEngine:
                              plan: Dict, violations: List, status: str):
         """Store reasoning session in database."""
         try:
+            # Ensure plan is serializable
+            plan_str = json.dumps(plan, ensure_ascii=False, default=str)
             data = {
                 'id': session_id,
                 'user_id': user_id,
                 'initial_request': request,
                 'context_summary': '',
-                'execution_plan_json': json.dumps(plan, ensure_ascii=False),
+                'execution_plan_json': plan_str,
                 'reasoning_steps_json': '[]',
                 'safety_violations_json': json.dumps(violations, ensure_ascii=False),
                 'final_decision_json': json.dumps({'status': status}),