feat: 新增 Agent、Memory、Skill 模块

- handler: agent_handler, memory_handler, skill_handler
- model: agent.go, skill.go
- repository: agent_repo, skill_repo
- service: agent_service, memory_service, skill_service
- 新增 migrations 目录

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/internal/model/skill.go

@@ -0,0 +1,28 @@
+package model
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+	"gorm.io/gorm"
+)
+
+// Skill 技能
+type Skill struct {
+	ID          string    `json:"id" gorm:"primaryKey"`
+	SkillName   string    `json:"skill_name" gorm:"size:200;not null"`
+	SkillType   string    `json:"skill_type" gorm:"size:20;not null"` // system / user
+	SkillDesc   string    `json:"skill_desc" gorm:"type:text"`
+	Path        string    `json:"path" gorm:"size:500"`               // skill 文件路径
+	Status      string    `json:"status" gorm:"size:20;default:'active'"`
+	CreatedAt   time.Time `json:"created_at"`
+	UpdatedAt   time.Time `json:"updated_at"`
+}
+
+// BeforeCreate 创建前自动生成ID
+func (s *Skill) BeforeCreate(tx *gorm.DB) error {
+	if s.ID == "" {
+		s.ID = uuid.New().String()
+	}
+	return nil
+}