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/adapter.go

@@ -105,9 +105,13 @@ func New(cfg Config, opts ...Option) *Adapter {
 			a.baseURL = defaultBaseURL
 		}
 	}
+	// KOOK authenticates bots with `Authorization: Bot <token>` (not "Bearer",
+	// which is reserved for OAuth2 user tokens). This is mandated by the KOOK
+	// reference: /doc/reference §鉴权.
 	a.http = httpclient.New(
 		httpclient.WithBaseURL(a.baseURL),
 		httpclient.WithToken(cfg.BotToken),
+		httpclient.WithAuthScheme("Bot"),
 		httpclient.WithHTTPClient(a.client),
 		httpclient.WithLogger(a.log),
 	)