chore: add empty changeset for docs-only skills sync

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: os-zhuang

.changeset/docs-skills-sync-9-4.md

@@ -0,0 +1,6 @@
+---
+---
+
+docs(skills): sync objectstack-* skills with 9.0→9.4 changes. Documentation-only
+update to the `skills/` authoring guides — no package version impact (empty
+changeset to satisfy the changeset check).