feat: LLM 调用日志 + ModelSelector 优化 + devtools.bat 编码修复

- 新增 call_log.go: 全局环形缓冲区记录每次 LLM 调用(模型/Token/耗时/错误)
- OpenAIProvider.doChat/ChatStreamWithTools 自动记录调用日志
- ai-core 暴露 GET /api/v1/llm-calls 端点, DevTools 代理 + UI 面板
- ModelSelector.envProvider 改为单例缓存, 避免重复创建 HTTP Client
- 新增 PurposeToolCalling 适配器, 后台思考工具调用走专用路由
- envFallback 超时 120s→180s, 显式设置 MaxRetries
- devtools.bat 全英文, 解决 Windows CMD GBK 编码乱码问题

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/ai-core/internal/llm/call_log.go

@@ -0,0 +1,74 @@
+package llm
+
+import (
+	"sync"
+	"time"
+)
+
+// CallRecord records a single LLM API call.
+type CallRecord struct {
+	Time             time.Time     `json:"time"`
+	Model            string        `json:"model"`
+	Duration         time.Duration `json:"duration_ms"`
+	PromptTokens     int           `json:"prompt_tokens"`
+	CompletionTokens int           `json:"completion_tokens"`
+	TotalTokens      int           `json:"total_tokens"`
+	Success          bool          `json:"success"`
+	Error            string        `json:"error,omitempty"`
+}
+
+// CallLogger is a thread-safe ring buffer for LLM call records.
+type CallLogger struct {
+	mu       sync.RWMutex
+	records  []CallRecord
+	capacity int
+	head     int
+	size     int
+}
+
+var globalCallLogger = &CallLogger{capacity: 500}
+
+// LogCall records an LLM call. Safe for concurrent use.
+func LogCall(r CallRecord) {
+	globalCallLogger.log(r)
+}
+
+// GetCalls returns recent call records, newest first.
+func GetCalls(limit int) []CallRecord {
+	return globalCallLogger.get(limit)
+}
+
+func (cl *CallLogger) log(r CallRecord) {
+	cl.mu.Lock()
+	defer cl.mu.Unlock()
+
+	if cl.records == nil {
+		cl.records = make([]CallRecord, cl.capacity)
+	}
+
+	r.Time = time.Now()
+	cl.records[cl.head] = r
+	cl.head = (cl.head + 1) % cl.capacity
+	if cl.size < cl.capacity {
+		cl.size++
+	}
+}
+
+func (cl *CallLogger) get(limit int) []CallRecord {
+	cl.mu.RLock()
+	defer cl.mu.RUnlock()
+
+	if limit <= 0 || limit > cl.size {
+		limit = cl.size
+	}
+
+	result := make([]CallRecord, limit)
+	for i := 0; i < limit; i++ {
+		idx := (cl.head - 1 - i) % cl.capacity
+		if idx < 0 {
+			idx += cl.capacity
+		}
+		result[i] = cl.records[idx]
+	}
+	return result
+}