chore(changeset): empty changeset — #3707 is comments and docs only

The Check Changeset gate requires every PR to add one; an empty-frontmatter
changeset is its own sanctioned "this PR releases nothing" declaration, which
is what a comment-and-docs change should say rather than minting a version bump
for prose. Same form as .changeset/adr-0104-design-doc-only.md.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

.changeset/control-plane-guard-crossref.md

@@ -0,0 +1,4 @@
+---
+---
+
+docs(audit): record that the control-plane surface is guarded from the `cloud` side (#3655) — comments and docs only, releases nothing.