Version: 0.5.8.dev.260315

♻️ refactor(agent): 拆分 agentsvc，并增强 quicknote/outbox 注释与可维护性

- 📦 将 Agent 服务实现从 `service` 根目录迁移到 `service/agentsvc`，包含 `agent.go`、`agent_quick_note.go` 及相关测试
- 🔌 新增 service 层兼容桥接 `agent_bridge.go`，保持 `service.NewAgentService` 与 `*service.AgentService` 现有调用方式不变
- 📝 为 `quicknote` 补充高密度中文步骤化注释，覆盖 `graph` / `runner` / `nodes` / `tool` / `state` / `prompt`，明确职责边界、分支条件、重试与兜底策略
- 🧭 为 `infra/outbox` 与 service agent 链路补充详细中文注释，覆盖状态机流转、幂等处理、失败回写与异步持久化语义
- ✅ 统一格式化相关文件，并通过全量后端测试：`go test ./...`

📝 chore(docs): 更新 AGENTS.md 注释强制规范

- 📚 追加“注释规范（强制）”与“注释风格示例”
- ✍️ 明确复杂逻辑必须使用步骤化注释、跨文件调用需写调用目的、注释需同步维护

backend/infra/kafka/producer.go

@@ -0,0 +1,45 @@
+package kafka
+
+import (
+	"context"
+	"errors"
+
+	segmentkafka "github.com/segmentio/kafka-go"
+)
+
+type Producer struct {
+	writer *segmentkafka.Writer
+}
+
+func NewProducer(cfg Config) (*Producer, error) {
+	if len(cfg.Brokers) == 0 {
+		return nil, errors.New("kafka brokers 未配置")
+	}
+	writer := &segmentkafka.Writer{
+		Addr:         segmentkafka.TCP(cfg.Brokers...),
+		Balancer:     &segmentkafka.Hash{},
+		RequiredAcks: segmentkafka.RequireOne,
+		Async:        false,
+	}
+	return &Producer{writer: writer}, nil
+}
+
+// Enqueue 将消息写入 Kafka。
+func (p *Producer) Enqueue(ctx context.Context, topic, key string, value []byte) error {
+	if p == nil || p.writer == nil {
+		return errors.New("kafka producer 未初始化")
+	}
+	msg := segmentkafka.Message{
+		Topic: topic,
+		Key:   []byte(key),
+		Value: value,
+	}
+	return p.writer.WriteMessages(ctx, msg)
+}
+
+func (p *Producer) Close() error {
+	if p == nil || p.writer == nil {
+		return nil
+	}
+	return p.writer.Close()
+}