fix(agent): render denied_agents list in access error

The f-string only spanned the first fragment ('You don') so the
{chr(44).join(...)} placeholder leaked into chat output as literal
text. Build the message with plain string concat.

agent_service/agents/master_agent.py

@@ -95,7 +95,8 @@ class MasterAgent:
                                       status='awaiting_clarification')
             access = await self._check_access(user_id, intent.agents)
             if not access.allowed:
-                msg = f'You don' + chr(39) + 't have access to: {chr(44).join(access.denied_agents)}.'
+                denied = ', '.join(access.denied_agents)
+                msg = "You don't have access to: " + denied + '.'
                 await self._memory.append_message(user_id, 'assistant', msg, directive_id)
                 await self._log_directive_complete(directive_id, 'failed', msg)
                 return MasterResponse(directive_id=directive_id, response=msg, status='failed')