Add Hidden to ToolDesc for reserved tool capabilities

Why: the fs tool family (proposal 2026-09-30-file-preview-and-reserved-
capabilities) is implemented but must stay invisible to models and tool
listings until a runtime gate opens it.

Evidence: new bool field with json omitempty; no golden samples carry it;
go test ./... green.

Impact: registries can mark gated tools; servers and runners filter on
it (backend PR in the same task).

Refs: task feature/file-preview-caps.

tools.go

@@ -20,6 +20,9 @@ type ToolDesc struct {
 	Source      SourceKind      `json:"source"`
 	PluginID    string          `json:"pluginId,omitempty"`
 	HTTP        *HTTPToolSpec   `json:"http,omitempty"` // 声明式 HTTP 工具（无代码）
+	// Hidden 预留能力形态：默认不对模型与 /api/tools 列表可见，须运行时开关
+	// （如 runtime.fsTools）或管理动作开启后可用（提案 2026-09-30-file-preview-and-reserved-capabilities）。
+	Hidden bool `json:"hidden,omitempty"`
 }
 
 // HTTPToolSpec：无代码自定义工具（4.8）。模板变量 {{param.x}} 从调用参数注入。