fix(kook): KOOK 机器人鉴权用 Authorization: Bot <token> 而非 Bearer

httpclient 硬编码 Bearer，导致 KOOK bot token 被当 OAuth2 用户令牌发，
/ gateway/index 返回 401『登录环境异常』。按 KOOK reference 鉴权规范，
给共享 httpclient 增加可配置 authScheme（默认 Bearer），KOOK 用 Bot。

- httpclient: 新增 WithAuthScheme 选项 + authScheme 字段，do() 按 scheme 组装
- kook/adapter: 装配时传 WithAuthScheme("Bot")
- 同步修正 config.go 注释与 send/identity 测试断言

plugins/channels/kook/send_test.go

@@ -38,8 +38,8 @@ func TestSendMessage_GroupText(t *testing.T) {
 	}
 	mu.Lock()
 	defer mu.Unlock()
-	if auth != "Bearer testtoken" {
-		t.Errorf("Authorization = %q, want Bearer testtoken", auth)
+	if auth != "Bot testtoken" {
+		t.Errorf("Authorization = %q, want Bot testtoken", auth)
 	}
 	if body["target_id"] != "chan-1" {
 		t.Errorf("target_id = %v, want chan-1", body["target_id"])