feat(v3): T17-T18 模型池 + 工具智能体后端

- T17 模型池：PoolStore（local/budget/premium 条目 + architect/worker/agent 角色指派），
  /pool CRUD+连通测试+模型探测端点；build_v2_pipeline 池指派优先（测试 override 最后）；
  V2Stats 新增 by_model 按 token/成本分账
- T18 智能体：OpenAI 兼容工具调用客户端（transport 可注入）+ AgentService
  （事件落盘 agent_runs/{id}/events.jsonl）+ /agent 提交/status/events/SSE stream
  + 工作区浏览/读取端点（越界 400）；轮数与 token 双护栏，模型经池 agent 角色或经典回退
- 新增测试 16 项，全量 262 passed（httpx 假注入，不依赖真实模型/key）

router_system/v2stats.py

@@ -22,6 +22,7 @@ class V2Stats:
         self._api_input_tokens = 0
         self._api_output_tokens = 0
         self._api_cost_usd = 0.0
+        self._by_model: Dict[str, Dict[str, Any]] = {}
         self._recent: List[Dict[str, Any]] = []
 
     def record(self, result) -> None:
@@ -38,6 +39,17 @@ class V2Stats:
             self._api_input_tokens += getattr(result, "api_input_tokens", 0)
             self._api_output_tokens += getattr(result, "api_output_tokens", 0)
             self._api_cost_usd += getattr(result, "cost_est", 0.0)
+            # 按模型分账（token / 成本 / 次数）
+            model = getattr(result, "model_used", None) or "unknown"
+            in_tok = getattr(result, "api_input_tokens", 0)
+            out_tok = getattr(result, "api_output_tokens", 0)
+            bucket = self._by_model.setdefault(model, {
+                "requests": 0, "input_tokens": 0, "output_tokens": 0, "cost_est_usd": 0.0,
+            })
+            bucket["requests"] += 1
+            bucket["input_tokens"] += in_tok
+            bucket["output_tokens"] += out_tok
+            bucket["cost_est_usd"] = round(bucket["cost_est_usd"] + getattr(result, "cost_est", 0.0), 6)
             self._recent.append({
                 "request_id": getattr(result, "request_id", ""),
                 "status": status,
@@ -69,6 +81,10 @@ class V2Stats:
                     "total": self._api_input_tokens + self._api_output_tokens,
                     "cost_est_usd": round(self._api_cost_usd, 6),
                 },
+                "by_model": {
+                    m: {**b, "cost_est_usd": round(b["cost_est_usd"], 6)}
+                    for m, b in self._by_model.items()
+                },
             }