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.
weichen