chore: add an empty changeset — this PR releases nothing

`Check Changeset` requires every PR to add a `.changeset/*.md` (diffed against the
PR base, so the directory's existing entries can't mask a missing one). An
empty-frontmatter changeset is the sanctioned "releases nothing" declaration, on par
with the `skip-changeset` label, and there is ample precedent for docs-only changes
(`adr-0104-design-doc-only.md` and others).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QoA8AV99Ss1RRkLLAYmDzq

Author: claude

.changeset/agents-spec-generated-artifacts-map.md

@@ -0,0 +1,13 @@
+---
+---
+
+Docs only — no package changes, nothing to release.
+
+AGENTS.md now maps each `packages/spec` change to the generated artifact it
+invalidates and the CI gate that catches it. The eight gates live in two jobs that
+run them sequentially, so the first stale artifact masks the rest — #4040 got one
+red build per artifact (a `.describe()` string via `check:docs`, then five new
+exports via `check:api-surface`) with no logic error involved either time.
+
+Also records the `check:api-surface` trap: it reads the built `dist/*.d.ts`, so a
+stale `dist` reports exports as REMOVED when nothing was removed.