fix: harden master agent synthesize/memory, fix expense create fields

- _synthesize: short-circuit on any single-agent report (avoids extra
  Ollama call that can timeout); wrap multi-agent LLM call in try/except
- _update_memory: catch exceptions so DB/memory failures don't kill reply
- _log_directive_start: use 0 instead of NULL for channel_id (NOT NULL col)
- create_expense: drop 'description' field (not valid on hr.expense in Odoo 18)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent_service/tools/expenses_tools.py

@@ -127,8 +127,6 @@ class ExpensesTools:
             vals['date'] = date
         if product_id:
             vals['product_id'] = product_id
-        if description:
-            vals['description'] = description
         return await self._o.create('hr.expense', vals)
 
     async def attach_receipt(self, model: str, record_id: int, filename: str,