feat: 支持 UserPromptSubmit 事件，将 prompt 存入 MySQL

- 新增 db.go：MySQL 连接管理、自动建表、兼容旧表 ALTER
- handler.go：处理 UserPromptSubmit，从 cwd 提取项目名称
- main.go：可选初始化 MySQL，优雅关闭释放连接
- config.go：新增 MYSQL_DSN 配置项
- 表结构：session_id + project_name + prompt + created_at

handler.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"log"
 	"net/http"
+	"path/filepath"
 )
 
 // ClaudeCodeHookEvent represents the JSON payload from Claude Code hooks
@@ -20,10 +21,11 @@ type ClaudeCodeHookEvent struct {
 
 type HookHandler struct {
 	config Config
+	db     *PromptDB // 可能为 nil，未配置 MySQL 时跳过数据库写入
 }
 
-func NewHookHandler(config Config) *HookHandler {
-	return &HookHandler{config: config}
+func NewHookHandler(config Config, db *PromptDB) *HookHandler {
+	return &HookHandler{config: config, db: db}
 }
 
 func (h *HookHandler) HandleHook(w http.ResponseWriter, r *http.Request) {
@@ -51,6 +53,11 @@ func (h *HookHandler) HandleHook(w http.ResponseWriter, r *http.Request) {
 		title, message, priority = h.handleStop(event)
 	case "PreToolUse":
 		title, message, priority = h.handlePreToolUse(event)
+	case "UserPromptSubmit":
+		h.handleUserPromptSubmit(event)
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprintf(w, `{"status":"ok"}`)
+		return
 	default:
 		log.Printf("ignored event: %s", event.HookEventName)
 		w.WriteHeader(http.StatusOK)
@@ -124,3 +131,27 @@ func (h *HookHandler) handleStop(event ClaudeCodeHookEvent) (title, message stri
 
 	return title, message, priority
 }
+
+// handleUserPromptSubmit 将用户提交的 prompt 写入数据库
+func (h *HookHandler) handleUserPromptSubmit(event ClaudeCodeHookEvent) {
+	if h.db == nil {
+		log.Printf("mysql not configured, skip saving prompt for session %s", event.SessionID)
+		return
+	}
+
+	prompt, _ := event.ToolInput["prompt"].(string)
+	if prompt == "" {
+		log.Printf("empty prompt in UserPromptSubmit, session=%s", event.SessionID)
+		return
+	}
+
+	// 从 cwd 提取项目名称（最后一级目录）
+	projectName := filepath.Base(event.CWD)
+
+	if err := h.db.SavePrompt(event.SessionID, projectName, prompt); err != nil {
+		log.Printf("failed to save prompt: %v", err)
+		return
+	}
+
+	log.Printf("prompt saved: session=%s, project=%s, len=%d", event.SessionID, projectName, len(prompt))
+}