feat(channel): KOOK 渠道适配器 + 框架兼容增强(wsutil/httpclient)

- plugins/channels/kook: config/adapter/gateway/websocket/send/message/errors — 实现 PlatformAdapter 13方法
  - WS 有状态网关: 取gateway地址→连WS→信令0/1/2/3/4/6 + 30s心跳/6s超时 + resume(拼接sn/session_id) + 指数退避重连
  - 收事件: 信令0 → UnifiedMessage(word注 channel_type/type/target_id/extra.author→字段)
  - 发消息: /message/create(频道) + /direct-message/create(私聊,修正正确端点,非user-chat) + Bearer鉴权
  - 注册 channels.go + CHANNELS_JSON 多账号 + channelbridge.configured 加 kook(bot_token非空才连)
- 框架增强: internal/channel/adapter/wsutil(连接管理/心跳/重连/resume骨架,复用gorilla,KOOK用上) + httpclient(Bearer+envelope归一,KOOK用)
- 全仓 go build/vet/test 绿, 零新依赖(复用餐gorilla), 未动telegram/qq/weixin/webhook契约
- §4.6 隔离/黑白名单/第三方边界未破坏

注: KOOK WS/WH 双模式待补(ReceiveMode), 实测需主人提供 CHANNEL_KOOK_TOKEN

plugins/channels/kook/errors.go

@@ -0,0 +1,17 @@
+package kook
+
+import "errors"
+
+// Sentinel errors returned by the KOOK adapter. Consumers test for them with
+// errors.Is; failures that mean "not connected" wrap adapter.ErrNotConnected so
+// the shared channel sentinel still matches.
+var (
+	// ErrMissingToken is returned when no BotToken is configured.
+	ErrMissingToken = errors.New("kook: missing bot token")
+	// ErrBadResponse is returned when the KOOK API replies with a non-zero code,
+	// an unexpected payload shape, or no usable gateway URL.
+	ErrBadResponse = errors.New("kook: bad api response")
+	// ErrNoChatTarget is returned when an outbound message carries no usable
+	// channel or user identifier.
+	ErrNoChatTarget = errors.New("kook: message has no target")
+)