fix(gateway): 收敛 sdk.Channel 与 adapter.PlatformAdapter 接口漂移，/admin/channels 恢复枚举

根因：sdk.Channel.Connect 为 2 参 (ctx, config any)，adapter.PlatformAdapter.Connect 为
1 参 (ctx)，Go 无重载 → 4 个渠道（qq-obv11/weixin-ilink/telegram/webhook）只实现 1 参
Connect，类型断言 cap.Value.(sdk.Channel) 恒失败 → /api/v1/admin/channels 恒返回 []。

收敛方向：sdk.Channel.Connect 改 1 参 (ctx)，与 PlatformAdapter 生命周期子集一致；
4 个真实适配器自动满足新接口，adminListChannels 枚举成功。最小改动（不改渠道实现、
不引依赖）。同步修正 sdk_test/registry_test/extra_test 的 mock。

pkg/plugins/sdk/channel.go

@@ -16,9 +16,9 @@ import "context"
 type Channel interface {
 	// Name returns the platform identifier (for example "qq", "telegram").
 	Name() string
-	// Connect establishes the platform connection using config. It may be
-	// called more than once and must be idempotent.
-	Connect(ctx context.Context, config any) error
+	// Connect establishes the platform connection. It may be called more than
+	// once and must be idempotent.
+	Connect(ctx context.Context) error
 	// Disconnect tears the connection down gracefully. It returns nil when the
 	// adapter was already disconnected.
 	Disconnect(ctx context.Context) error

pkg/plugins/sdk/sdk_test.go

@@ -31,7 +31,7 @@ func (s stubSkill) Guardrails() []Guardrail {
 type stubChannel struct{ name string }
 
 func (s stubChannel) Name() string                           { return s.name }
-func (s stubChannel) Connect(_ context.Context, _ any) error { return nil }
+func (s stubChannel) Connect(_ context.Context) error { return nil }
 func (s stubChannel) Disconnect(_ context.Context) error     { return nil }
 func (s stubChannel) IsConnected() bool                      { return true }
 func (s stubChannel) HealthCheck(_ context.Context) error    { return nil }