feat: implement mid-platform core services and APIs [AC-IDMP-01~20, AC-MARH-01~12]
- Add dialogue, messages, sessions, share API endpoints
- Add mid-platform schemas and models (memory, tool_registry, tool_trace)
- Add core services: agent_orchestrator, policy_router, runtime_observer
- Add tool services: kb_search_dynamic, memory_adapter, tool_registry
- Add guardrail services: output_guardrail_executor, high_risk_handler
- Add utility services: timeout_governor, segment_humanizer, metrics_collector