chore: add empty changeset for the docs-only regeneration

The `ui/action` reference doc regeneration is generated output and releases
nothing on its own — the `@objectstack/spec` release is declared by the
changeset in #3406. This empty-frontmatter changeset satisfies the
"Check Changeset" gate, which diffs added .changeset files against this PR's
base (the #3406 head) and otherwise sees none.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011W4a1eVS3aaYVRxKut68hK

Author: claude

.changeset/regenerate-ui-action-reference-doc.md

@@ -0,0 +1,7 @@
+---
+---
+
+docs(spec): regenerate the `ui/action` reference doc for the inline lookup
+`reference` key. Generated output only — the release for this change is
+declared by the `@objectstack/spec` changeset in #3406, so this PR itself
+releases nothing.